Mr Biden's family discussed the future of his campaign when they met for a long-planned photo shoot at Camp David in Maryland on Sunday, and told him he should "keep fighting", NBC, Sky News' US partner said, quoting two sources familiar with the discussions.

His wife Jill Biden and Mr Biden's son, Hunter, the people whose opinions he values most, both insisted he should stay in the race as he's the best-placed candidate to defeat Donald Trump, the presumptive Republican nominee.

It was a view echoed by the rest of the family, who instead blamed the president's advisers for failing to prepare him properly.

Around 40 of Biden's top financial backers were briefed by campaign manager Julie Chavez Rodriguez, who told them most of the campaign's significant war chest would fall to Vice President Kamala Harris, with a smaller pool of money kept by the Democratic National Committee (DNC).

It was one of several meetings in recent days in which campaign managers have tried to shore up support for Mr Biden, whom, they insist, can still win November's election.

But a Biden victory appeared a long way away as the 81-year-old Democrat candidate paused and stumbled his way through a gruelling TV debate on Thursday in Atlanta, his performance dismissed as an "unmitigated disaster" by some in his own party.

A poll of polls maintained by analysts FiveThirtyEight had the pair roughly neck and neck until the debate, but Donald Trump has since pulled ahead, if only by one percentage point. RealClearPolitics put Trump 1.9 percentage points in front.

Watched by an estimated 51 million people, Mr Biden, whose age was already seen as a liability by voters, sounded raspy, trailed off, and at times gave convoluted answers during the debate.

The president's showing sparked alarm among supporters, donors and campaign surrogates, worried that Mr Biden's age, already targeted by Trump, 78, had caught up with him, and this was one campaign too far.

Mr Biden's campaign has been working hard to persuade them that is not the case ever since.

Prominent Democrats made a public show of support for him on Sunday, among them congressman James Clyburn of South Carolina, who said: "I do not believe that Joe Biden has a problem leading for the next four years."

Read more from Sky News:
New York Times tells Biden to quit
Joe Biden: Could the Democrats replace him?
Who is Gavin Newsom? Democrat who could replace Joe Biden
Biden performance among worst in presidential history

? Click to subscribe to the Sky News Daily wherever you get your podcasts ?

But former senator Tom Harkin called the debate "a disaster from which Biden cannot recover" and Democratic congressman Jamie Raskin said there were "serious and rigorous conversations taking place" over replacing Biden, according to NBC.

Democratic National Convention (DNC) chairman Jaime Harrison and Mr Biden's campaign manager, Julie Chavez Rodriguez, spoke to some of the most influential members of the party on Saturday, presenting a rosy outlook and taking no questions.

Multiple Democrat officials on the call, most speaking on condition of anonymity, said it felt like they were being asked to ignore a serious predicament.

Joe Salazar, an elected DNC member from Colorado, said they were being "gaslit".

It comes as a YouGov poll for CBS News found that the percentage of respondents who felt that Mr Biden did not have the mental and cognitive health required to be president was now 72%, up from 65% in the previous poll carried out in early June, before the debate.