Washington DC - Joe Biden fended off questions about his mental fitness and electability on Friday in a TV interview that failed to draw a line under the disastrous debate performance that triggered calls for him to quit his re-election bid.

With rebellion brewing among some nervous Democratic voters, lawmakers and donors, the one-to-one with ABC's George Stephanopoulos was hyped as the most consequential of the 81-year-old president's long career.

But the 22-minute sit-down appeared to provide little reassurance, as Biden blamed sickness for his sub-par performance and repeatedly dismissed polling and fears within his party that the debate had gravely wounded his prospects.

"I was sick, I was feeling terrible... I just had a really bad cold," said the president, in his first unscripted remarks of any length since last week's showdown with Republican rival Donald Trump.

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The interview was intended to help Biden weather the storm, but his hoarse voice and meandering answers drew immediate fire from David Axelrod, a top aide in Barack Obama's White House, for appearing "dangerously out-of-touch."

Asked if staying in the race might jeopardize Democrats holding on to the White House, Biden said: "I don't think anybody's more qualified to be president or win this race than me."

He dodged calls to have doctors assess his mental acuity, saying that the duties of the presidency mean "I have a cognitive test every single day. Every day I have that test, everything I do."

The Trump campaign mockingly posted on social media that "Biden sounds great" as the broadcast went out, before slamming the president as "in denial and in decline."

Biden's campaign has been pushing back hard on any suggestion he may withdraw and, just hours before the ABC interview, released an aggressive campaign travel schedule for the rest of July.

Appearing at a campaign rally in Madison, Wisconsin, the president delivered an energetic stump speech, unequivocally declaring, "I'm staying in the race. I'll beat Donald Trump."

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Post-debate polls have shown a widening deficit in Trump's favor, and at least four Democrats in Congress have called on Biden to step aside, as have major newspapers, donors and a raft of Democratic-supporting political commentators.

Senate Intelligence Committee chairman Mark Warner and House Minority Leader Hakeem Jeffries were both planning crisis talks with lawmakers in the coming days, reports said.

In Friday's interview, Stephanopoulos repeatedly referenced the growing Democratic clamor for a conversation on picking a new candidate, asking Biden if he would step down if convinced he couldn't beat Trump.

"Well, it depends. If the Lord Almighty comes down and tells me that, I might do that," he said.

Biden added that he had not watched the debate afterward, saying "I don't think I did, no."

And he dismissed the idea that his poor performance was a sign of a more serious health problem.

"It was a bad episode, no indication of any serious condition. I was exhausted. I didn't listen to my instincts in terms of preparing and – and I had a bad night," Biden said.