US President Joe Biden has used a highly anticipated TV interview to declare that only the “Lord almighty” could oust him from the race, even as pressure from Democrats intensifies and more donors threaten to cut funding.

Biden also repeatedly refused to take an independent medical examination to prove to voters that he is capable of a second term.

“Look, I have a cognitive test every single day,” Biden told ABC’s George Stephanopoulos, referring to the tasks he faces daily in a rigorous job.

“Every day, I have that test. Everything I do. You know, not only am I campaigning, but I’m running the world.”

The 81-year-old Biden made it through the 22-minute interview on Saturday (AEDT) without any major blunders that would inflict further damage to his struggling candidacy.

But it appeared unlikely to fully tamp down concerns about his age and fitness for another four years, as well as his ability to defeat Donald Trump in November.

On Saturday, another Democratic lawmaker joined close to a half dozen in saying Biden should not run again.

Representative Angie Craig of Minnesota said that given what she saw and heard in the debate, and Biden’s “lack of a forceful response” afterward, he should step aside “and allow for a new generation of leaders to step forward”.

Craig secured one of the Democrats’ key suburban wins in the 2018 mid-terms and could be a barometer for districts that were vital for Biden in 2020.

Senator Mark Warner, a well-respected moderate Democrat, is inviting Democratic senators to a meeting on Monday (US time) to discuss Biden’s campaign, a source told Reuters.

The Washington Post reported Warner was seeking to ask the group to press Biden to exit the race.

The president told reporters later he had spoken to at least 20 lawmakers and they were telling him to stay.

Asked about Warner calling for him to go, Biden said: “Well, Mark Warner, to my understanding, is the only one considering that.”

During the interview, Biden insisted he was not more frail than earlier in his presidency.

He said he undergoes “ongoing assessment” by his personal doctors and they “don’t hesitate to tell me” if something is wrong.

“Can I run the 100 in 10 flat? No. But I’m still in good shape,” Biden said.

As for the debate: “I didn’t listen to my instincts in terms of preparing,” Biden said.

Biden suggested that Trump’s disruptions — from just a few feet away — had flustered him: “I realised that, even when I was answering a question and they turned his mic off, he was still shouting and I let it distract me. I’m not blaming it on that. But I realised that I just wasn’t in control.”

Meanwhile, many influential donors like the co-founder of Netflix, the Disney heiress, and cryptocurrency titan Mike Novogratz have threatened to cut off funding to the Democratic Party, having lost confidence in Biden for a second term.

The list of donors turning their backs on Biden includes billionaire businessmen such as Gideon Stein, president of the Moriah Fund, who recently announced suspending $US3.5 million ($5.2 million) in donations to NGOs and political groups allied with the Democrats.

“Virtually every major donor I have talked to believes that we need a new candidate in order to defeat Donald Trump,” Stein told CNBC.

Hollywood is also unconvinced about Biden: producer Damon Lindelof and Endeavor CEO Ari Emanuel, two critical voices in entertainment, publicly urged other donors to suspend funding until another Democratic candidate emerges.

The Walt Disney Company heiress Abigail Disney and Netflix co-founder Reed Hastings also disapproved of the president’s candidacy and cut off their million-dollar funding.

At a breakfast on Friday in Colorado with about 50 influential donors and the political action committee American Bridge, a major source of opposition research and paid campaigns against Donald Trump, attendees were asked to raise their hands if they thought Biden should step aside.

Almost all raised their hands, according to The New York Times.

-with AAP