Poor debate was due to 'bad night' not serious condition, Biden says

US President Joe Biden has blamed his disastrous debate performance against Republican rival Donald Trump on a "bad night," vowing that he would not drop out of the race as calls for him to do so grow.

In his first televised interview since last week's debate with ABC News on Friday, Biden described the evening as a "bad episode," but said there was "no indication of any serious condition."

Pressure on Biden to step aside as the Democrats' 2024 presidential candidate has been growing from within his own ranks since he faced off against Trump at a 90-minute televised debate on June 27.

During the showdown, a raspy-voiced Biden fumbled his sentences, seemed to lose his train of thought and often struggled to get his words out.

Asked by ABC News' George Stephanopoulos about his bad performance, Biden said "I was feeling terrible" and that he had had "a really bad cold" that day.

"I was exhausted. I didn't listen to my instincts in terms of preparing and ... and a bad night."

'I'm staying in the race'

Biden insisted earlier on Friday that he would not drop his bid for a second term.

"Let me say this as clearly as I can, I'm staying in the race. I'll beat Donald Trump," he forcefully declared to a crowd of chanting supporters at a campaign event in the state of Wisconsin.

The fallout from the debate was quick and severe as a chorus of political pundits declared his campaign to be in deep trouble. In the days since, Democrats have been in crisis mode amid new opinion polls showing 81-year-old Biden losing more ground to 78-year-old Trump.

"I learned long ago, when you get knocked down, you get back up," Biden said at the rally in Madison, Wisconsin. "I'm not letting one 90-minute debate wipe out three-and-a-half years of work."

"Let me ask you: You think I'm too old to beat Donald Trump?" Biden, who is the oldest president in US history, asked the cheering crowd.

"No!" they yelled back.

"I can hardly wait," the president said.

Biden confident on fitness

In Friday's televised interview, ABC's Stephanopoulos continued to press Biden on the state of his physical and mental health as the president appeared to be evading questions on the issue several times.

"Are you sure you are being honest with yourself when you say you have the mental and physical capacity to serve another four years?" Stephanopoulos asked.

"Yes, I am," Biden said. "The last thing I want to do is not be able to meet that."

When asked whether he would be willing to undergo an independent medical examination that includes a cognitive test and release the results to the public, Biden declined.

"I have a cognitive test every single day. Every day I test everything I do. You know, not only am I campaigning, I'm running the world," the president said.

Asked whether it was worth taking the risk to remain in the race, with polls suggesting he is behind, Biden said: "I don't think anybody's more qualified to be president or win this race than me."