Defiant Biden says he doesn't need a cognitive test

US President Joe Biden says he does not need a cognitive test to determine whether he has a mental decline as he dismissed calls to step aside in the presidential race.

A defiant Biden, scrambling to defuse the political crisis over his shaky debate performance, used a 22-minute ABC News interview to argue he was the right candidate to beat Donald Trump.

Under persistent questioning, Biden dismissed polls that showed him trailing Trump and said other surveys have him in a better position.

Joe Biden
Biden attended a rally in Wisconsin and recorded a television interview in a busy day of campaigning (AP PHOTO)

Asked if he had undergone a specific cognitive test for a neurological exam, Biden, 81, said he had not had one and "no one said I had to".

Asked if he would undergo a cognitive test, he was dismissive, citing the rigours of the White House as sufficient.

"Look, I have a cognitive test every single day. Every day I have that test – everything I do (is a test)," Biden said.

In his first interview since the debate, Biden said he was the best candidate to take on Trump and that only "Lord almighty" could oust him from the race.

Biden cited "a really bad cold" for his stumbling performance at the debate and exhaustion after two trips to Europe.

"I was exhausted. I didn't listen to my instincts in terms of my preparing. It was a bad night," he told interviewer George Stephanopoulos.

Biden said he was distracted to some degree by Trump talking when his microphone was shut off during the debate.

"I let it distract me. I realised that I just wasn't in control," he said.

Kamala Harris (R) at a Fourth of July celebrations
Vice President Kamala Harris (right) is favourite to replace Biden should he leave the contest. (EPA PHOTO)

Before the interview, Biden offered a defiant defence of his determination to stay in the race at a Wisconsin rally.

"We had a little debate last week. Can't say it was my best performance. But ever since then there's been a lot of speculation. 'What's Joe gonna do? Is he gonna stay in the race? Is he gonna drop out?" Biden said.

"Well here's my answer: I am running and gonna win again."

Senator Mark Warner, a well-respected moderate Democrat, is inviting Democratic senators to a meeting on Monday 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."

Vice President Kamala Harris has emerged as a top choice to replace him were he to step aside as the Democratic Party's standard-bearer.

Harris posted a supportive note on X, formerly Twitter, saying the president had devoted his life to fighting for Americans.

"In this moment, I know all of us are ready to fight for him," she said.

But Illinois Democratic Representative Mike Quigley called on Biden to step aside and "let someone else do this" or risk "utter catastrophe".

Biden knocked Trump's intelligence and called him a liar, delivering stinging attacks that were absent when he appeared on the Atlanta debate stage.

He referenced a comment in which Trump mistakenly said George Washington's revolutionary army had taken over British airports in 1776.

"He is a 'stable genius,", Biden said.

Trump's campaign and some of his allies have launched a pre-emptive political strike on Harris, moving swiftly to try to discredit her.

Trump, 78, who made multiple false statements during the debate, falsely claimed in a video that was circulated on social media he had driven Biden out of the race.

He made disparaging comments about Harris in the same video, which the Trump campaign stood by.