US President Joe Biden is set to welcome NATO allies to Washington DC for a summit that pundits say could sink or save his run for the presidency.

Biden, 81, will spend three days meeting alliance world leaders on an international stage, giving the Democrat the chance to convince allies at home and abroad he can still lead.

It comes as Democrat politicians huddled behind closed doors on Tuesday (US time) for their own meeting amid mounting fears about Biden’s prospects in the November 5 presidential election.

Party members exited the meeting unable to reach consensus on whether to rally behind Biden or force him to stand down.

When asked if Democrats were on the same page, Congressman Steve Cohen said: “We’re not even in the same book.”

More than a week after the disastrous Biden-Donald Trump debate, Biden continues to face daily questions about his fitness for office and a punishing election campaign.

His health was again the focus of the latest White House press conference after intense questioning the previous day about a Parkinson’s doctor’s repeated visits to the White House.

Press secretary Karine Jean-Pierre said Biden was not being treated for Parkinson’s, and defended the administration’s responses to questions about his health.

Jean-Pierre reiterated that Biden was “more determined than ever to get the job done”. He was eager to “turn the page” on the issue of his suitability to run for president again, she said.

“We want to get to the other side of this. We want to continue doing the work and that’s what the president is going to do,” she said.

The week’s events in Washington DC will give Biden a chance to address the concerns, including a high-profile speech on Tuesday and a rare solo press conference on Thursday (US time).

Worried Democrats meet

While just six House of Representatives Democrats have publicly called for Biden to step aside, more have voiced concerns about his election chances.

The deepening rift within the party has sent the Biden campaign scrambling to contain further defections.

On Monday, Biden told MSNBC 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 US representative Joe Morelle said constituents in his New York state district told him over the July 4 holiday weekend they were losing confidence in Biden following his poor June 27 debate performance against Republican challenger 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 ease concerns.

Even senior lawmakers who are backing 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, adding Biden “must seriously consider the best way to preserve his incredible legacy”.

Biden has vowed to remain in the race, arguing Trump, 78, poses a unique threat to democracy.

Trump, who repeated multiple falsehoods during the debate, repeatedly and falsely clams his 2020 loss was due to fraud and has not committed to accepting the 2024 election results.

Democratic lawmakers also worry that Biden’s struggles could damage their chances of capturing a majority in the House, which be the party’s sole bulwark against Trump should he win in November.

Republicans currently hold a 220-213 majority.

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 wanted the party to unite on a strategy for the campaign by week’s end, whether Biden remained on the ticket or not.

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 saying, “As I’ve said before, I’m for Joe.”

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

However, the poll also found that none of his possible replacements fared better in a match-up against Trump.

The poll showed Biden and Trump tied at 40 per cent each.

-with AAP