Joe Biden faced fresh turmoil Wednesday as a heavyweight Democrat urged him to abandon his reelection bid and a backlash grew over plans for a virtual nomination before the party conference.
California Representative Adam Schiff urged Biden to "pass the torch", becoming one of the highest profile Democrats to do so and the first since the assassination attempt against Donald Trump.
The key congressional powerbroker spoke out shortly before Biden was to make his latest bid to shore up his candidacy, with a speech courting Latino voters in the battleground state of Nevada.
The 81-year-old Biden has been fighting for political survival since a disastrous debate against Donald Trump nearly three weeks ago, in which his tired and confused appearance sparked concerns about his age.
"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.
The drumbeat of Democrats calling for Biden to step aside was briefly silenced by the attempt on Republican rival Trump's life on Saturday, but has now started up again in earnest.
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 in recent weeks aimed at showing he has what it takes.
He has blamed the debate on illness and jetlag, while his campaign says a series of recent verbal flubs are par for the course for a man famous for gaffes during a political career lasting more than four decades.
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.
Vice President Kamala Harris is considered the frontrunner if Biden does drop out, although he has shown no sign he will.
Party chiefs say they need to carry out the virtual roll-call by August 7, which is the deadline set by 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 J.D. Vance.
While Ohio's governor has signed a law giving Biden more time, the DNC said it feared further legal challenges.
The DNC's rules committee said in a letter to lawmakers obtained by AFP that "no virtual voting will begin before August 1," adding that it aimed to make a final decision next week.
"None of this will be rushed," it said. "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 criticized it, according to US media.
Biden insists that Democratic voters support him, but a poll by the Associated Press and NORC Center for Public Affairs Research said Wednesday that nearly two-thirds want him to step aside.
