WASHINGTON — Democrats will begin building a public case on Wednesday that President Trump committed extortion, bribery or coercion by trying to enlist Ukraine to help him in the 2020 elections, as they open the first presidential impeachment hearings in more than two decades.

With a bitterly divided nation watching with anticipation, Democrats will gavel open a nationally televised hearing that features two veteran diplomats as star witnesses, and seeks to breathe life into their allegations that Mr. Trump’s dealings with Ukraine constitute high crimes and misdemeanors for which he deserves to be removed.

“It’s a calm day, it’s a prayerful day, it’s a solemn day for our country,” said Speaker Nancy Pelosi, Democrat of California. “It’s a sad day which I wish we never had to face.”

On the eve of the session, which will unfold across from the Capitol in an ornate wood-and-plasterwork room adorned with blue and gold, Democrats leading the impeachment investigation described Mr. Trump’s conduct in the starkest terms yet, signaling a new, more aggressive approach.

A top Democratic official said the witnesses — William B. Taylor Jr., the top diplomat in Ukraine, and George P. Kent, a senior State Department official — would lay out a timeline of serious misconduct by Mr. Trump and describe how the president sought to “bribe, extort, condition or coerce” the leader of another country. The official spoke on condition of anonymity without authorization to publicly describe internal strategy, but the language echoed the definition in the Constitution of behavior that warrants impeachment and removal from office.

Representative Adam B. Schiff, the California Democrat who is chairman of the House Intelligence Committee, promised that the two witnesses, who still serve in Mr. Trump’s administration, will reveal the president’s actions to be “a corrupt undertaking that is evident from his own words.”

In a taste of the epic partisan battle to come, Republicans readied their arguments that the president did nothing wrong — and certainly nothing impeachable — and raged against a process they have denounced from the beginning as unfair and illegitimate.

“We want to make sure the truth gets out,” said Representative Kevin McCarthy of California, the minority leader. “There is no reason for the president to be in this impeachment.”

House investigators have spent seven weeks methodically assembling evidence through closed-door interviews that Mr. Trump used security aid as leverage to force President Volodymyr Zelensky of Ukraine to publicly announce investigations into former Vice President Joseph R. Biden Jr. and debunked claims that Democrats conspired with Ukraine to interfere in the 2016 election.

Mr. Taylor and Mr. Kent will be the first of several witnesses to testify publicly. The next is Marie L. Yovanovitch, the former United States ambassador to Ukraine, who is to appear Friday and is expected to tell her story of being smeared by Mr. Trump and his allies based on lies and conspiracy theories, and abruptly removed from her post.

Late Tuesday, the committee announced three more days of public hearings next week involving eight witnesses, signaling Democrats’ intent to accelerate the public phase of the inquiry ahead of the Thanksgiving holiday.

Four current or former officials will testify next Tuesday, including two requested by Republicans: Tim Morrison, a former Russia expert on the National Security Council, and Kurt D. Volker, who was the president’s special envoy to Ukraine. Jennifer Williams, a national security aide who worked for Vice President Mike Pence, and Alexander S. Vindman, a decorated lieutenant colonel in the Army who worked at the National Security Council, will also testify.

Gordon D. Sondland, the ambassador to the European Union, will be the most prominent witness next Wednesday. Other witnesses have testified that he helped lead the rogue effort to pressure Ukraine. He will be joined that day by David Hale, a top State Department official, and Laura K. Cooper, a senior Defense Department official.

Democrats will close out next week by hearing testimony from Fiona Hill, a former top national security aide in the White House, who testified in a closed-door session that John R. Bolton, the former national security adviser, had told her to report concerns about the activities of the president’s political appointees to the National Security Council’s lawyers.

Mr. Schiff said Tuesday evening that the witnesses would tell a compelling story of a president who deployed his inner circle, including his personal lawyer Rudolph W. Giuliani, to make sure that “American policy toward Ukraine was subverted to serve the president’s personal, political interests, not the national interest.”

In an interview on Tuesday, Mr. Schiff told NPR that he believed Mr. Trump had committed “bribery” and “high crimes and misdemeanors,” both listed in the Constitution as impeachable offenses.

Republicans pledged to aggressively defend Mr. Trump against the charges by arguing that his efforts to push Mr. Zelensky to announce investigations into his political rivals were legitimate attempts to root out corruption. The president did it, they plan to concede, but for innocent reasons.

According to a memo circulated on Tuesday by their top impeachment investigators, Mr. Trump’s Republican defenders on Capitol Hill plan to raise doubts about the witnesses who have testified in the Ukraine affair, questioning the knowledge and judgment of the diplomats and national security officials who have expressed concern about Mr. Trump’s July 25 telephone call with Mr. Zelensky.

“Democrats want to impeach President Trump because unelected and anonymous bureaucrats disagreed with the president’s decisions and were discomforted by his telephone call with President Zelensky,” the memo’s author’s wrote. “The president works for the American people. And President Trump is doing what Americans elected him to do.”

According to the memo, Republicans will insist that the president had a “deep-seated, genuine and reasonable skepticism” about Ukraine given its history of corruption, and that his decision to withhold security aid was “entirely reasonable.” The detailed description of the strategy was distributed to the party’s members in the House late Monday night by the Republican staff of the committees conducting the impeachment inquiry.

Wednesday’s session in the vaulted, columned chambers of the Ways and Means Committee — the House’s grandest meeting room — will be only the third time in modern American history that lawmakers have convened a public hearing to consider the impeachment of a president.

Democrats promised to conduct a sober, dignified inquiry, but the vast potential consequences of their endeavor promised to turn the process into an ugly partisan fight that is all but certain to eclipse the facts.

Lawmakers from both parties on the Intelligence Committee spent hours Tuesday sequestered in separate rooms of the panel’s secure bunker beneath the Capitol, ironing out scripts for proposed questioning.

Democratic officials acknowledged that Wednesday’s hearing was unlikely to produce new, blockbuster revelations that go beyond what witnesses have told investigators behind closed doors. But they said their main goal was to allow Americans to hear the evidence for themselves.

“The new thing for the American people to see is the caliber and the character of these witnesses,” said Representative Jim Himes of Connecticut, the second-ranking Democrat on the committee. “They are just superb, articulate, patriotic people. And that obviously creates some challenges for the Republicans. The president’s go-to defense is to delegitimize people he sees as a threat.”

A Democratic aide working on the inquiry said it would be up to Republicans to provide evidence that would clear Mr. Trump of the accusations against him.

Representative Jim Jordan, Republican of Ohio, emerged from his party’s strategy session in the secure Intelligence Committee rooms surrounded by a cadre of aides toting fat legal briefing books and what appeared to be blown-up printouts of exhibits for Wednesday’s hearing.

The people closest to Mr. Trump, including Mr. Bolton and Mick Mulvaney, the acting White House chief of staff, have refused to testify so far in the inquiry. The White House says those aides are covered by “absolute immunity,” which Mr. Trump’s lawyer argues allows them to ignore congressional demands for testimony.

On Tuesday, Mr. Mulvaney said he would follow the directive from Mr. Trump and refuse to testify, abandoning an attempt to join a lawsuit that could have clarified whether he was legally compelled to obey the president or Congress.

Republicans have signaled that they intend to use the hearings as a platform to shift the discussion away from Mr. Trump and toward a series of largely debunked conspiracy theories about Democrats, including the business dealings of Hunter Biden while his father, who is now a Democratic candidate for president, was vice president.

But the memo circulated Monday night indicates that the president’s allies will try to defend the president’s actions toward Ukraine as well. It urges lawmakers to argue that Mr. Trump’s July 25 call with Mr. Zelensky shows “no conditionality or evidence of pressure” on the president of Ukraine. That is at odds with a reconstructed transcript of the call, which shows that when Mr. Zelensky brought up his desire to purchase defense weaponry, Mr. Trump said he wanted the Ukrainian president to “do us a favor, though” and then mentioned the investigations into the Bidens.

The memo also makes a broader case that there was no pressure on Mr. Zelensky because the Ukrainians were not aware that the United States was withholding promised security assistance, and that the aid was ultimately released without Ukraine committing to the investigations that Mr. Trump wanted.

In fact, officials have testified that some Ukrainian officials were aware that the aid had been frozen by the Trump administration, and were eventually told by a top American diplomat that they would probably not receive it unless the investigations were publicly announced. The release of the aid occurred only after the Trump administration knew that a whistle-blower had filed a confidential complaint about the July 25 call. That complaint alleged that the freezing of the aid was part of a broader abuse of power by Mr. Trump in an effort to better his political chances in 2020.

Hours after the Republican memo was made public, Democrats released to their members a memo of their own calling Republicans’ assertions “patently false” and challenging them point by point.

“The American people will hear firsthand from these witnesses beginning this week,” the memo to Democratic members said.

Sheryl Gay Stolberg contributed reporting.