WASHINGTON (Reuters) - Democrats in the U.S. Congress were due to huddle behind closed doors on Tuesday amid mounting fears about their prospects in the Nov. 5 election, after President Joe Biden defiantly rejected calls by some in his party to end his campaign.

While just a half-dozen House of Representatives Democrats have publicly called for the 81-year-old incumbent to step aside and allow someone else to face Republican Donald Trump, several more have voiced concerns about Biden's chances after a halting debate performance raised fresh questions about his ability to mount a successful campaign - and to keep up with a grueling job for another 4-1/2 years.

The deepening rift within the party has sent the Biden campaign scrambling to contain further defections. The president told MSNBC on Monday by phone that he was "not going anywhere," a message he repeated to donors on a private call later in the day, according to two sources on the call.

Biden made multiple campaign stops on Sunday in the battleground state of Pennsylvania. Vice President Kamala Harris, seen as the likeliest candidate to succeed Biden were he to step aside, has also been campaigning for the president.

But Democratic U.S. Representative Joe Morelle told reporters that constituents in his New York state district told him over the July 4 holiday they were losing confidence in Biden following his poor June 27 debate performance against Trump.

"They're going to need more proof to feel secure in the knowledge that he can continue to do the job. And so telling them that isn't going to work. He'll have to demonstrate it," said Morelle, who added that more public events where Biden answered questions from voters could help assuage their concerns.

Even senior lawmakers who were supportive of Biden say he needs to do more.

"We need to see a much more forceful and energetic candidate on the campaign trail in the very near future," Democratic Senator Patty Murray, chair of the powerful Senate Appropriations Committee, said in a statement on Monday night, adding that Biden "must seriously consider the best way to preserve his incredible legacy."

Biden has vowed to remain in the race, arguing that Trump, 78, poses a unique threat to democracy. Trump, who repeated multiple falsehoods during the debate, has falsely claimed that his 2020 loss was the result of fraud and has not committed to accepting this year's results.

Democratic lawmakers, particularly in the House, also worry that Biden's struggles could damage their chances of capturing a majority in that chamber, which could serve as Democrats' sole bulwark against Trump should he prevail. Republicans currently hold a 220-213 majority in the House.

'DECISION IS THE PRESIDENT'S'

Democrats face a far tougher path to protect their 51-49 Senate majority, as they are defending multiple seats in Republican-leaning states.

Democratic Senator Michael Bennet said he wants Democrats to unite on a strategy for the campaign by week's end - whether Biden remains on the ticket or not.

"What I hope to see is, over the course of this week, our coming together on the kind of compelling and successful path forward that the American people need," the Colorado Democrat told reporters.

Former Democratic House Speaker Nancy Pelosi, when asked whether Democrats should stick with Biden, said, "People should be prayerful, thoughtful. And the decision is the president's. It's not the caucus's."

Other prominent Democrats have voiced confidence in Biden, with Senate Majority Leader Chuck Schumer telling reporters, "As I've said before, I'm for Joe."

The chair of the Congressional Black Caucus, Representative Steven Horsford, expressed support for Biden's candidacy on Monday. Black voters are a critical part of Democrats' base of support.

A Reuters/Ipsos poll last week found that one in three registered Democratic voters believed that Biden should quit the race, with 59% saying he is too old to work in government.

However, the poll also found that none of his possible replacements fared better in a matchup against Trump. The poll showed Biden and Trump tied at 40% each.

"I hope he continues to reach out to voters the way that we saw him this weekend, talking to them unscripted," said Senator Ben Ray Lujan of New Mexico. "The more that he does that, I think the more support that we're going to see across the country."

(Reporting by Richard Cowan, Moira Warburton and David Morgan, additional reporting by Allende Miglietta; writing by Joseph Ax; Editing by Scott Malone and Himani Sarkar)

By Richard Cowan, David Morgan and Moira Warburton