US President Joe Biden said he could drop his reelection bid if doctors found he had a medical condition, as a top Democrat on Wednesday urged the 81-year-old to step aside.

Biden's comments were the first time he has even slightly opened the door to abandoning the White House race, and came as Representative Adam Schiff, a key ally from California, urged Biden to "pass the torch."

"If I had some medical condition that emerged, if somebody, if the doctors came and said 'you've got this problem, that problem,'" Biden told the Black media outlet BET in an interview recorded on Tuesday, when asked what could make him rethink.

Biden however defended his decision to stay on for a rematch with Republican Donald Trump in November, and explained why he had not handed over to a younger generation after one term.

"I said I was going to be a transitional candidate, and I thought I'd be able to move on from this, pass it on to someone else," he said. "But I didn't anticipate things getting so, so, so divided."

Biden has been fighting for political survival since a disastrous debate against Trump nearly three weeks ago, in which his tired and confused appearance sparked concerns about his age.

Schiff became the first Democrat to call for him to step aside since the assassination attempt against Trump on Saturday, which had briefly silenced the growing chorus against Biden.

"A second Trump presidency will undermine the very foundation of our democracy, and I have serious concerns about whether the President can defeat Donald Trump in November," Schiff said in a statement to the Los Angeles Times.

Schiff, who is expected to win a Senate seat this November, is a key White House ally in the legislature and shot to nationwide prominence as lead prosecutor during then-president Trump's first impeachment trial.

Biden was set to make a fresh attempt to prop up his candidacy in a speech to crucial Latino voters in the battleground state of Nevada later on Wednesday.

Around 20 House Democrats and one senator have now called on Biden to leave the White House race but Biden has refused, insisting he is best placed to beat Trump.

Most polls show Biden trailing in a tight race, with Trump pulling ahead in key swing states but no dramatic movement since the debate debacle or shooting.

Biden said his mental acuity was "pretty damn good" in an NBC interview on Monday, one of a series of unscripted outings aimed at showing he has what it takes.

With pressure on Biden mounting, Democrats said on Wednesday they plan a virtual nomination for the president in the first week of August, ahead of the Democratic National Convention (DNC) on August 19.

Some Democrats have slammed the scheme, accusing the party of trying to ram through Biden's candidacy and avoid a full discussion of alternative choices.

Party chiefs say they need to carry out the virtual roll-call by August 7, which is the deadline set by the Republican-led state of Ohio for the submission of nominations.

Biden otherwise risks not being on the ballot in Ohio, the home state of Trump's new running mate JD Vance.

While Ohio's governor has signed a law giving Biden more time, the DNC said it feared further legal challenges.

"None of this will be rushed," the heads of the DNC's rules committee said in a letter to lawmakers. "No matter what may be reported, our goal is not to fast-track."

But several lawmakers are planning to sign a letter against the virtual nomination plan and others have criticised it, according to US media.

Biden insists that Democratic voters support him, but a poll by the Associated Press and NORC Centre for Public Affairs Research said on Wednesday that nearly two-thirds want him to step aside. (AFP)