President Joe Biden's top campaign staffer insisted on Friday that he would stay in the race even as calls grew among his fellow Democrats in Congress for him to end his floundering reelection bid and make way for a new candidate.

More than one in ten congressional Democrats have now publicly called on the 81-year-old incumbent -- who is isolating at his Delaware home with a case of COVID -- to drop out following a disastrous June debate against Republican Donald Trump that raised questions about Biden's ability to win or to carry out his duties for another four years.

The divide among Democrats stood in sharp contrast to the scenes that played out his week at the Republican National Convention in Milwaukee, where former party rivals united around Trump, who accepted the Republican nomination in a rambling speech that featured his familiar mix of grievance and bombast.

So far, 28 of the 264 Democrats in Congress have openly called for Biden to end his campaign, while other senior Democratic leaders have pushed him behind the scenes to do so, according to sources and media reports.

"We must face the reality that widespread public concerns about your age and fitness are jeopardizing what should be a winning campaign," Representatives Jared Huffman, Marc Veasey, Chuy Garcia and Marc Pocan wrote in a letter on Friday.

Democratic U.S. Senator Martin Heinrich said on social media: "While the decision to withdraw from the campaign is President Biden's alone, I believe it is in the best interests of our country for him to step aside."

Democrats are increasingly worried about a Republican sweep in the Nov. 5 election that could leave Trump and his allies not only in charge of the White House but also with majorities in both chambers of Congress.

Biden had no public events scheduled for Friday, but a top adviser said he would return to the campaign trail next week.

"He is absolutely in it," Biden campaign chairwoman Jen O'Malley Dillon told MSNBC in an interview.

Former House of Representatives Speaker Nancy Pelosi is among those who have told Biden he cannot win in November, according to a White House source familiar with the matter.

After weeks of insisting he will remain in the race, Biden is now taking calls to step aside seriously, and multiple Democratic officials think an exit is a matter of time, sources said.

Though a Reuters/Ipsos poll earlier this week found Biden and Trump effectively tied, strategists from both parties say Biden's path to victory is narrowing as he trails in most of the battleground states that will decide the election.

Were Biden to step aside as a candidate, Vice President Kamala Harris, 59, could fill the role. Reuters/Ipsos polling shows her as performing marginally better against Trump in a theoretical head-to-head matchup.

A Democratic Party committee was due to meet Friday to discuss a virtual voting process to formally nominate their candidate ahead of the convention to begin Aug. 19. Critics within the party have argued it is a means to lock the embattled incumbent in early.