Joe Biden's presidential reelection bid hung in the balance Saturday, after his latest efforts to put a disastrous debate showing behind him failed to silence voices urging him to quit the White House race.

Murmurs of dissent within his own Democratic Party have -- in the case of five individual House representatives -- morphed into direct calls for him to drop out. And a number of key donors have threatened to cut off funding if Biden insists on staying the course.

"I do not believe that the President can effectively campaign and win against Donald Trump," Angie Craig, the latest House Democrat to break ranks, said Saturday.

The House minority leader, Hakeem Jeffries, has scheduled a virtual meeting of senior Democrat representatives for Sunday to discuss the best way forward, and Democrat Senator Mark Warner is reportedly working to convene a similar forum in the upper chamber.

In what had been billed as a make-or-break TV interview on Friday, Biden's strategy was to flatly deny the falling poll numbers and concerns over his mental and physical fitness triggered by his dismal performance against rival Donald Trump.

He blamed a severe cold for the debate debacle and insisted it was just a "bad night" rather than evidence of increasing frailty and cognitive decline.

And the 81-year-old was adamant that he would not be pressured to end his campaign.

"If the Lord Almighty came down and said, 'Joe, get out of the race,' I'd get out of the race," he said. "But the Lord Almighty is not coming down."

Calls for less divine intervention do, however, appear to be strengthening.

Biden's campaign team is pushing ahead regardless, with two events planned in Pennsylvania on Sunday and visits to other battleground states later in the month.

At a rally in Wisconsin before Friday's interview, Biden had delivered a forceful, energetic stump speech, unequivocally declaring, "I'm staying in the race. I'll beat Donald Trump."

Then came the sit-down with the ABC network that appeared unlikely to soothe the concerns of critics who say that -- away from a teleprompter -- Biden can struggle to communicate.

Some of his answers were tentative, meandering and difficult to follow, even as he sought to deflect questions about his mental acuity and dismissed the notion that his party would consider replacing him.

The Biden campaign had another small fire to put out Saturday after it emerged that the White House had provided the questions for interviews the president gave to two Black radio stations on Friday.

Democratic strategist David Axelrod suggested in a CNN op-ed that Biden is engaged in "Denial. Delusion. Defiance."

"The stakes are as great as Biden describes. And if he believes it, as I think he does, he will eventually do what duty and love of country requires, and step aside," Axelrod wrote in the piece published Saturday.

"If he does not, it will be Biden's age, and not Trump's moral and ethical void, that will dominate the rest of this most important campaign and sully the president's historic legacy."

Trump, meanwhile, sarcastically suggested Biden should "ignore his many critics and move forward, with alacrity and strength."

"He should be sharp, precise and energetic, just like he was in The Debate," the Republican challenger said in a social media post Saturday.

For now, Democrats are largely keeping a lid on any simmering discontent with their leader -- at least in public.

But with election day just four months away, any move to replace Biden as nominee would need to be made sooner rather than later, and the meetings of top Congressional Democrats in coming days will be scrutinized for any signs of more open rebellion.

Meanwhile, for Biden and his campaign team, the strategy seems to be to ride it out.

His next major test will be a press conference scheduled for Thursday during the NATO leaders' summit in Washington.

When pressed in the ABC interview on why he doesn't take an independent neurological exam, Biden argued that the role of US president meant being subjected to constant mental assessment.

"I have a cognitive test every single day," he said. "Not only am I campaigning, I'm running the world." (AFP)