Washington DC - President Joe Biden is set to deliver a "big boy" press conference on Thursday, his first major appearance since his debate disaster against Donald Trump, that will be a make-or-break moment for his teetering re-election bid.

The eyes of the world will be on the 81-year-old's solo appearance at a NATO summit in Washington as concerns over his age and health spark growing calls in his Democratic Party for him to step aside.

Biden will be under close scrutiny to show he can handle a rare unscripted exchange with reporters.

Any missteps by Biden at the event – which was delayed by an hour to 6:30 PM – could turn the trickle of Democrats who have so far urged him to abandon his 2024 election bid into a flood.

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The dam appeared to be breaking on Wednesday when Hollywood actor and Democratic supporter George Clooney called on Biden not to stand, and party grandee Nancy Pelosi stopped short of backing him.

Around 14 Democratic members of the House of Representatives have openly urged the man who beat Trump four years ago to drop out, along with one Democratic senator.

A new poll released on Thursday showed more than half of Democrats say Biden should end his bid for a second term, and two-thirds of Americans believe he should quit the race.

But the debate did not seem to have moved the overall battle with Trump, with the former president and the incumbent remaining in a dead heat on 46%, according to the Washington Post-ABC News-Ipsos survey.

Biden's campaign was, however, quietly testing the strength of Vice President Kamala Harris in a potential match-up against Trump, The New York Times reported.

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Some of the president's longtime aides are meanwhile discussing how to persuade him that he should step aside, the paper also said, citing anonymous sources.

The White House said the report was "unequivocally" false.

Harris has firmly stood behind Biden and was out campaigning for him in North Carolina on Thursday.

"We always knew this election would be tough, and the past few days have been a reminder that running for president of the United States is never easy," Harris said.

"But one thing we know about our president, Joe Biden, is that he is a fighter."

Biden now faces what CNN called the "most high-pressure presidential news conference in modern history," exactly two weeks after his stumbling debate performance ignited a firestorm of concerns about his age, health and mental acuity.

White House Press Secretary Karine Jean-Pierre has repeatedly promised Thursday's "big boy press conference" – a phrase first used by a journalist that she has since adopted – will feature multiple questions.