Madison, Wisconsin - A defiant Joe Biden told supporters Friday he would stay in the White House race and beat Donald Trump, as he readied for a crucial TV interview to persuade doubters of his physical and mental fitness for a second term.

With his re-election bid in peril and an apparent rebellion from some nervous Democratic donors, the one-to-one with the ABC network has been hyped as the most consequential of the 81-year-old's long career.

It comes after Biden's disastrous campaign debate last week with Donald Trump triggered panic within his Democratic Party and calls for him to pull out of the race.

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

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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."

As supporters cheered, he went on the attack against his rival.

"Let's focus on what really matters," Biden said, reading from teleprompters. "We're running against the biggest liar and the biggest threat... to our democracy in American history – that's not hyperbole."

Post-debate polls have shown Biden's pool deficit widening, and at least three of his party members in Congress have now called on him to step aside, as have several major newspapers and a raft of Democratic-supporting political commentators.

Wealthy Disney heiress and Democratic supporter Abigail Disney told CNBC she plans to withhold donations to the party until Biden drops out, saying bluntly that "if Biden does not step down, the Democrats will lose" in November.

"This is realism, not disrespect," she said.

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Biden has not spoken publicly without a teleprompter since the debate, other than some brief remarks.

Friday's interview with ABC presenter George Stephanopoulos, set to be recorded in Wisconsin, will be a key moment for Biden to try to dispel the worries and reset expectations.

With anticipation soaring, ABC has switched up its original plan of airing excerpts through the weekend and will instead broadcast the interview in full on Friday at 8:00 PM ET.

Biden will face an experienced interviewer known for his firm, effective style.

Stephanopoulos worked for former Democratic president Bill Clinton during his first campaign and was one of his closest advisors during his first term in the White House.

Biden will have to win back confidence with a clear and cohesive performance after he was often incoherent in the debate.

Former US House speaker Nancy Pelosi, one of the most influential Democratic voices, has deemed it "essential" that he do more major interviews to prove himself.

The White House has announced he will hold a press conference during the NATO summit in Washington next Thursday but has not revealed any details on its format or length.

Biden's travel plans include Pennsylvania this weekend, then the NATO meeting, and back on the campaign trail with a swing through the American Southwest.

The president will "engage in frequent off-the-cuff moments over the course of the month, as he has consistently throughout this campaign," his team said, seeking to emphasize his folksy charm.

Trump has meanwhile dared Biden to another debate, or an "all-on discussion," saying he was ready "anytime, anywhere, any place."

With speculation swirling over a potential switch in the Democratic ticket, Kamala Harris, the nation's first female vice president, has suddenly been in the spotlight.

The 59-year-old former California prosecutor – who joined Biden at Thursday's July 4 celebrations – has been performing a delicate balancing act since the debate.

She has offered unwavering support for Biden in public but is standing by as a leading contender to replace him if he steps aside.