Rep. Pramila Jayapal from Washington, the chair of the Congressional Progressive Caucus, announced her backing for Harris after the two spoke on the phone Sunday. 

"I told her I am 1000% in for her to be our President! She has the smarts, the experience, the accomplishments and the agenda to lead us to victory in November. Let's go!" she wrote on social media. 

Pennsylvania Gov. Josh Shapiro endorsed Harris Sunday evening, calling it the "best path forward" for the Democratic Party.

In a statement praising her service, he said she's "ready to be President."

"The best path forward for the Democratic Party is to quickly unite behind Vice President Harris and refocus on winning the presidency," Shapiro said. "The contrast in this race could not be clearer and the road to victory in November runs right through Pennsylvania – where this collective work began. I will do everything I can to help elect Kamala Harris as the 47th President of the United States."

Shapiro, who was viewed as a potential contender for the nomination in the wake of Mr. Biden's departure from the race, also said he has spoken with both Mr. Biden and Harris. 

Hunter Biden expressed gratitude for his father for his decades of public service, even as he experienced so much heartbreak through the deaths of his first wife and daughter in a car crash and later his son from brain cancer. 

"For my entire life, I've looked at my dad in awe," the president's son said in a statement. "How could he suffer so much heartache and yet give so much of whatever remained of his heart to others." 

"Over a lifetime I have witnessed him absorb the pain of countless everyday Americans who he's given his personal phone number to, because he wanted them to call him when they were hurting. When their last hopes were slipping through their hands," the statement said. "That unconditional love has been his North Star as a president, and as a parent. He is unique in public life today in that there is no distance between Joe Biden the man and Joe Biden the public servant of the last 54 years. I'm so lucky every night I get to tell him I love him, and to thank him. I ask all Americans to join me tonight in doing the same. Thank you, Mr. President. I love you, Dad." 

Trump said in a social media post that he does not believe ABC News should host the next presidential debate, which is scheduled for Sept. 10.

Trump and Mr. Biden agreed to participate in two debates: The first, hosted by CNN, was held June 27, and ABC News said it would be holding the second in September.

Harris agreed to participate in a vice presidential debate hosted by CBS News, but the Trump campaign had not agreed on a date or network host for that debate. 

"My debate with Crooked Joe Biden, the Worst President in the history of the United States, was slated to be broadcast on Fake News ABC, the home of George Slopadopolus, sometime in September," Trump posted. "Now that Joe has, not surprisingly, has quit the race, I think the Debate, with whomever the Radical Left Democrats choose, should be held on FoxNews, rather than very biased ABC."

Delegates and party leaders are being told the Democratic roll call vote is expected to take place on Aug. 1, according to two delegates and a Democratic source in touch with the Biden campaign. The DNC has yet to confirm the date publicly, but has indicated it will provide more information soon. 

By Ed O'Keefe, Aaron Navarro, Jonah Kaplan (WCCO)

CBS News' Robert Costa spoke on the phone with Trump to get his reaction to news of the president dropping out. The former president called it a shock to the country and said he wasn't sure if Harris would be the Democratic nominee. Watch:

California Rep. Adam Schiff urged Democrats to unite behind Harris and said the party must focus its energy on helping her beat Trump.

"There is no better matchup than this superb former prosecutor against this convicted criminal, and she'll prosecute the case against Trump every single day," Schiff said in a statement.

The California Democrat is running for the Senate and expected to win in November. He was one of the highest profile House Democrats to push Mr. Biden to end his campaign for reelection.

Schiff echoed the sentiments of many of his fellow Democrats who have said Mr. Biden has accomplished more in his one term in office than many presidents do in two terms.

"Joe Biden has always put the country first and he did so again today, after more than 50 years of tremendous service," he said.

Schiff praised Harris as having the "experience, leadership and tenacity" to defeat Trump in the general election.

Biden for President, the president's principal campaign committee, filed paperwork with the Federal Election Commission changing its name to "Harris for President."

"NOTE: Committee Name is different than previously reported," the new statement of organization says, naming Kamala Harris as the candidate.

South Carolina Rep. Jim Clyburn said in a statement that he is supporting Harris' candidacy for the Democratic nominee for president. Clyburn is one of Mr. Biden's closest allies on Capitol Hill and had remained steadfast in his support for the president amid the calls for him to exit the race for the White House.

Referencing Mr. Biden's selection of Harris as his running mate in 2020, Clyburn said he echos "the good judgment he demonstrated in selecting Vice President Harris to lead this nation alongside him, and I am proud to follow his lead in support of her candidacy to succeed him as the Democratic Party's 2024 nominee for president."

The South Carolina Democrat also lauded Mr. Biden for his "unparagoned leadership" and said he has brought "the American people together through incredible trials and unfamiliar terrain."

Clyburn was instrumental in helping Mr. Biden secure the Democratic nomination in 2020, lending him his support before the crucial South Carolina primary.

Alan Solomont, a former U.S. ambassador to Spain and a major fundraiser for the Biden campaign who serves on its national finance committee, told CBS News he believes Harris is "best equipped" to defeat Trump. He said people have been contacting him and that he's been telling them "it's time to fall in behind the vice president and the person I believe will become the Democratic nominee."  

He called for the kind of unity and discipline he's seen in the Republican Party "to save this country from falling into the abyss." Solomont also said he's received emails from donors about contributing to Harris and predicted that "we're going to see a surge in support for Vice President Harris." Solomont also said his pick for VP for Harris would be Arizona Sen. Mark Kelly.

Trump's running mate, Ohio Sen. JD Vance, reacted to Mr. Biden's decision and Harris' potential nomination, saying "she owns" the "failures" of the Biden administration. He also accused Harris of lying about the president's mental acuity.

"Joe Biden has been the worst President in my lifetime and Kamala Harris has been right there with him every step of the way," Vance wrote. "Over the last four years she co-signed Biden's open border and green scam policies that drove up the cost of housing and groceries. She owns all of these failures, and she lied for nearly four years about Biden's mental capacity — 

saddling the nation with a president who can't do the job. President Trump and I are ready to save America, whoever's at the top of the Democrat ticket. Bring it on." 

An emotional Sen. Chris Coons of Delaware, one of the president's closest allies, told CBS News that the president made a "very difficult decision" to "step aside in terms of the nomination of my party for president in order to make certain that Donald Trump is not returned to the White House."

He said he does not expect any serious challengers to Harris to emerge, and called on the party to come together behind her bid for the nomination. 

"Today I think we should focus on the legacy of Joe Biden as president. I do recognize the urgency of the moment as we head to our convention in Chicago in August. The most important thing we can do is come together," Coons said. "Vice President Harris has an incredible legacy of success and accomplishment as a senator and vice president. I do not expect a serious challenge to her nomination."

In her first statement since Mr. Biden withdrew from the race, Harris officially said she will be seeking the Democratic presidential nomination and intends to win it.

"I am honored to have the President's endorsement and my intention is to earn and win this nomination. Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election. And that is what I will continue to do in the days and weeks ahead. I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump and his extreme Project 2025 agenda," she wrote.

She thanked Mr. Biden for his "extraordinary leadership," adding that it is a "profound honor" to serve as his vice president. 

"His remarkable legacy of accomplishment is unmatched in modern American history, surpassing the legacy of many Presidents who have served two terms in office," she said. "With this selfless and patriotic act, President Biden is doing what he has done throughout his life of service: putting the American people and our country above everything else."

Robert F. Kennedy Jr., the independent running for president, issued a statement on social media saying that he felt Mr. Biden's "infirmities were evident to any unbiased observer from the beginning. It was this progressive deterioration — and his abandonment of Democratic Party principles — that prompted me to enter the race and ensure American voters had a viable, vigorous alternative to Donald Trump. "

Kennedy said he felt the party "should use neutral polling to identify the candidate who can best beat Donald Trump" rather than "anointing" a candidate chosen by "DNC elites."

"The delegates should then select a nominee based on this information," Kennedy said. "If they had done this to begin with, I would not have had to leave the Democratic Party."

The Biden campaign and the Democratic National Committee are holding an all-hands staff call at 5 p.m., as they scramble to determine next steps. 

The DNC is supposed to have a virtual roll call for the nomination in August. But the Biden campaign and DNC have much to work out in the meantime, if that schedule holds. 

It's not known whether the president or vice president will join the party's call. 

— Ed O'Keefe, Fin Gomez, Aaron Navarro 

Former President Barack Obama released a lengthy statement that commended Mr. Biden as "a patriot of the highest order" and "one of America's most consequential presidents." 

"I also know Joe has never backed down from a fight," Obama said. "For him to look at the political landscape and decide that he should pass the torch to a new nominee is surely one of the toughest in his life. But I know he wouldn't make this decision unless he believed it was right for America. It's a testament to Joe Biden's love of country  —  and a historic example of a genuine public servant once again putting the interests of the American people ahead of his own that future generations of leaders will do well to follow." 

Obama did not endorse Harris in the statement. He said the Democratic Party "will be navigating uncharted waters in the days ahead," but he has "extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges." 

"I believe that Joe Biden's vision of a generous, prosperous, and united America that provides opportunity for everyone will be on full display at the Democratic Convention in August," Obama said. "And I expect that every single one of us [is] prepared to carry that message of hope and progress forward into November and beyond."

Mr. Biden consulted family and allies, but he made the decision to drop out largely alone and in isolation, sources close to the president tell CBS News chief election and campaign correspondent Robert Costa. 

Those sources say the weight on the president's shoulders has been enormous, and he's felt extreme pressure as he has been isolated with COVID at his Delaware beach home since Wednesday night. 

The president's family members have told him he needs to do what's best for him, those sources said. But the president kept his ultimate choice very close to the vest. 

The president's decision ultimately came together with just three to four of his closest people in the last six to 12 hours, the sources said.

Rep. Nanette Barragán, the chair of the Congressional Hispanic Caucus, said on social media that she backs Harris as the Democratic presidential nominee and pledged to "work tirelessly to make sure she is elected as our next president in November."

Barragán, of California, also thanked Mr. Biden for his "lifetime of service to this country & the American people."

The president had sought to ensure the continued support of the Congressional Hispanic Caucus while he rebuffed the earlier calls to exit the presidential race. The group's political arm, BOLD PAC, announced Friday that it would be endorsing Mr. Biden.

Sen. Patty Murray of Washington state, who serves as the president pro tempore of the Senate, joined the growing list of Democrats throwing their support behind Harris as the party's presidential nominee.

"We must beat Donald Trump — and I know Kamala Harris can win. I am behind Vice President Harris 100% — she is exactly the woman we need to prosecute the case against Donald Trump, save American democracy, lead the fight to restore abortion rights, and build an economy that puts working people — not billionaires — first," Murray said in a statement. "I will do everything I can to help elect Kamala Harris as our next president."

Murray was the first woman to serve as the Senate's president pro tempore and has been a vocal advocate of abortion rights. She also offered thanks to Mr. Biden and said she has a "profound respect" for his willingness to exit the presidential race.

"America owes a deep debt of gratitude to Joe Biden for his historic leadership—a true patriot who loves this country," she said.

Sen. Elizabeth Warren of Massachusetts backed Harris for president, calling her a "proven fighter who has been a national leader in safeguarding consumers and protecting access to abortion." 

"As a former prosecutor, she can press a forceful case against allowing Donald Trump to regain the White House," she said in a statement. "We have many talented people in our party, but Vice President Harris is the person who was chosen by the voters to succeed Joe Biden if needed. She can unite our party, take on Donald Trump, and win in November." 

She also praised Mr. Biden's presidency as "transformational" and said "he deserves full credit for beating Donald Trump in 2020. 

"His selfless decision today gives us our best shot at doing it again in 2024," she said. 

"While Donald Trump tried to tear down our democracy to maintain his grip on power, Joe Biden willingly stepped aside in order to protect our democracy. President Biden's selfless action is a profound gift to the people of the United States — and it's on all of us not to waste it." 

Former President Bill Clinton and Secretary of State Hillary Clinton joined Mr. Biden in endorsing Harris for the Democratic presidential nomination and said it's time for the party to fight to make sure she is elected.

"We are honored to join the president in endorsing Vice President Harris and will do whatever we can to support her," the Clintons said in a statement. 

Citing Trump's comment that he would only be a "dictator" on his first day back in office and the Supreme Court's recent ruling on presidential immunity, the Clintons said they are concerned about the threat posed by a second term with Trump in office.

"Now is the time to support Kamala Harris and fight with everything we've got to elect her. America's future depends on it," they said.

The Clintons also offered praised for Mr. Biden for what they said is an "extraordinary career" in public service.

"We join millions of Americans in thanking President Biden for all he has accomplished, standing up for America time and again, with his North Star always being what's best for the country," they said.

Alan Clendenin of Florida, a member of the DNC's executive committee, previewed the path forward for the party, telling CBS News that the nomination is in the hands of the Democratic delegates.

"The delegates at the convention are the ones who place our candidate on the ballot. We will convene in Chicago. I'm not sure if the electronic voting is going to happen yet — whether it's that roll call ahead of time or in Chicago," Clendenin said. "We will have the procedures that have been outlined for two years now. The delegates will cast the vote."

He predicted that "a supermajority of folks are going to coalesce around the vice president," and said "the more interesting conversation is who her vice president could be."

House Speaker Emerita Nancy Pelosi praised Mr. Biden, calling him "a patriotic American who has always put our country first. His legacy of vision, values and leadership make him one of the most consequential Presidents in American history." 

"God blessed America with Joe Biden's greatness and goodness," she added. 

Mr. Biden's withdrawal from the race for the White House marks the first time in decades an incumbent president has dropped their reelection bid. The most recent and closest precedent was in 1968, when President Lyndon B. Johnson declined to run for the Democratic nomination.

Johnson's announcement on March 31 kicked off a tumultuous process to find a replacement that ended in Vice President Hubert Humphrey becoming the nominee at the party's convention in Chicago.

This year's Democratic National Convention will also take place in Chicago and is set to begin Aug. 19.

House Democratic Leader Hakeem Jeffries praised Mr. Biden's record of public service and said the country is "forever grateful" to him.

"President Joe Biden is one of the most accomplished and consequential leaders in American history," Jeffries, of New York, said in a statement. "In less than one term, he rescued the nation from a once-in-a-century pandemic, brought the economy roaring back from the brink of recession, enacted consequential legislation for everyday Americans and saved our democracy by defeating the Insurrectionist-in-Chief."

The House's top Democrat said the country is a better place today because of Mr. Biden's leadership.

In a call with Democratic officials regarding credentials for the upcoming convention, Democratic National Committee chairman Jamie Harrison said that he was "emotional" and "still riding with Biden." 

"I just want to say this on a personal level," he said. "I am emotional about the president's decision. Because this President Joe Biden has been a transformational president. He's been a great leader. He's a good man, a decent man who has done so much for this nation, but so much to see us as to value us to fight for us." 

He continued: "I'm emotional, because I am still riding with Biden. I still support my president and we will get through this, my friends, as we always do." 

In a social media post, California Gov. Gavin Newsom offered words of praise for Mr. Biden. Newsom has been mentioned as a possible presidential hopeful, though he remained supportive of the president amid the drumbeat of calls for him to leave the 2024 race.

"President Biden has been an extraordinary, history-making president — a leader who has fought hard for working people and delivered astonishing results for all Americans. He will go down in history as one of the most impactful and selfless presidents," he wrote.

Newsom closed with thanks for Mr. Biden.

Mr. Biden's decision to exit the presidential race came after more than three dozen Democrats in Congress had publicly urged him to step aside. The calls began in the days following the June 27 debate with Trump, and continued into Sunday morning, when Sen. Joe Manchin of West Virginia and Rep. Dean Phillips of Minnesota joined the chorus of Democrats urging Mr. Biden to withdraw. Manchin is an independent who still caucuses with Democrats in the Senate.

Here is the full list of House and Senate Democrats who had openly called on the president to end his campaign.

In a social media post, California Gov. Gavin Newsom offered words of praise for Mr. Biden. Newsom has been mentioned as a possible presidential hopeful, though he remained supportive of the president amid the drumbeat of calls for him to leave the 2024 race.

"President Biden has been an extraordinary, history-making president — a leader who has fought hard for working people and delivered astonishing results for all Americans. He will go down in history as one of the most impactful and selfless presidents," he wrote.

Newsom closed with thanks for Mr. Biden.

The decision to step aside before the Democratic convention, which begins on Aug. 19 in Chicago, is legally easy but politically difficult, according to Derek Muller, a law professor at the University of Notre Dame who specializes in election law. 

The decision on who would replace Mr. Biden on the ticket is left to the delegates at the Democratic convention, the National Task Force on Election Crises notes. 

Because he stepped aside before the convention, Mr. Biden will probably have significant influence on the Democratic Party's choice of who would take his place on the ticket. He is backing Harris. 

If Harris were to become the presidential nominee before the convention, she would be able to pick her running mate, perhaps another big-name Democrat who performed well in an open convention. 

Read more about the process here. 

House Speaker Mike Johnson called on Mr. Biden to resign from office and accused Democrats of nullifying the votes of millions of Americans who voted for the president during the primary process. 

"If Joe Biden is not fit to run for President, he is not fit to serve as President," he said in a statement. "He must resign the office immediately. November 5 cannot arrive soon enough."

He called the decision an "unprecedented juncture in American history." 

"We must be clear about what just happened," he said. "The Democrat Party forced the Democrat nominee off the ballot, just over 100 days before the election. Having invalidated the votes of more than 14 million Americans who selected Joe Biden to be the Democrat nominee for president, the self-proclaimed 'party of democracy' has proven exactly the opposite."

Mr. Biden's granddaughter, Naomi Biden, took to social media to offer praise for the president, saying the world is better today because of him.

"I'm nothing but proud today of my Pop, our President, Joe Biden, who has served our country with every bit of his soul and with unmatched distinction," she wrote. "Not only has he been — and will continue to be — the most effective president of our lifetime, but he has likely already cemented himself as the most effective and impactful public servant in our nation's history."

Vice President Kamala Harris has emerged as a top contender to replace President Biden at the top of Democratic ticket following his historic decision to withdraw from the 2024 race for the White House.

Mr. Biden roiled the general election when he announced Sunday that he would be stepping aside, ending weeks of pressure from within the Democratic Party that began after his shocking performance at the first presidential debate on June 27.

Read more about Harris, who has been endorsed by Mr. Biden for the Democratic presidential nominee.

In a post to his social media platform, former President Donald Trump, the 2024 Republican presidential nominee, lambasted Mr. Biden.

"Crooked Joe Biden was not fit to run for president, and is certainly not fit to serve - And never was!" Trump wrote. "He only attained the position of President by lies, Fake News, and not leaving his Basement."

In a second post, he wrote that Mr. Biden "was not fit to serve from the very beginning, but the people around him lied to America about his Complete and Total Mental, Physical, and Cognitive Demise. Whoever the Left puts up now will just be more of the same."

Senate Majority Leader Chuck Schumer shared the following statement after President Biden announced his decision to drop out of the race:

"Joe Biden has not only been a great president and a great legislative leader but he is a truly amazing human being. His decision of course was not easy, but he once again put his country, his party, and our future first.

"Joe, today shows you are a true patriot and great American."

President Biden announced his decision to end his campaign in a statement posted on social media:

"While it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as president for my term," Mr. Biden wrote.

In another post a few minutes later, he wrote: "My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as President for the remainder of my term. My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And it's been the best decision I've made. Today I want to offer my full support and endorsement for Kamala to be the nominee of our party this year. Democrats — it's time to come together and beat Trump. Let's do this."

Mr. Biden said he would address the nation later this week.