Washington — Senate Democrats will be meeting with top advisers to President Biden Thursday amid calls for the White House to do more to reassure the party about the president's path to reelection and fitness for office after his performance in last month's debate. 

At a special caucus lunch meeting, the senators will hear from senior advisers to the president Mike Donilon, the president's longtime speechwriter, and Steve Ricchetti, counselor to the president, along with Biden campaign chair Jen O'Malley Dillon, a Senate Democratic leadership aide told CBS News. 

The president has been fighting to prove he's up for another four years on the job as lawmakers returned to Capitol Hill with varying levels of support and doubt related to Mr. Biden's ability to win reelection and serve a second term. The president worked to assuage concerns early in the week, telling lawmakers in a letter that he's "firmly committed" to running and making his case for reelection. But that hasn't stopped what's been a slow drip of Democrats calling for Mr. Biden to leave the race in recent days. 

In the upper chamber, where Mr. Biden represented Delaware for nearly three decades, senators have been more reticent. Some have limited their public responses to stating the president needs to do more to assure voters and the party that he's up for a second term. Until late Wednesday, no senators had called for Mr. Biden to step aside. 

Sen. Peter Welch of Vermont, who expressed frustration with the Biden campaign in recent days and concern about Mr. Biden's reelection prospects, called for the president to drop out of the race Wednesday night, becoming the first senator to do so. 

In an op-ed in the Washington Post, Welch wrote that while he understands why the president wants to run, having defeated former President Donald Trump in 2020 and aiming to do it again, Welch said that "he needs to reassess whether he is the best candidate to do so."

"In my view, he is not," Welch wrote. "For the good of the country, I'm calling on President Biden to withdraw from the race."

Also on Wednesday, former House Speaker Nancy Pelosi said "it's up to the president" to decide if he's going to run, suggesting that the decision remains an open question, despite Mr. Biden's insistence that only the Lord Almighty would get him to drop his reelection bid. 

Then, actor George Clooney, a major Democratic donor, penned an op-ed calling on the president to step aside. Two more House Democrats called on the president to drop out of the race on Wednesday, bringing the number to nine. 

Amid the growing concerns from House Democrats, House Minority Leader Hakeem Jeffries plans to convey the concern of his caucus directly to the president, multiple sources confirm to CBS News. Jeffries has been meeting with Democratic groups in the caucus this week. 

Jeffries told CBS News on Wednesday that House Democrats are "continuing to have candid and clear-eyed and comprehensive conversations with the House Democratic Caucus throughout the week," adding that "we'll see where we go from there."

Ed O'Keefe, Nikole Killion and Ellis Kim contributed reporting.