Representative Adam Schiff on Wednesday became the most heavyweight Democrat so far to publicly urge US President Joe Biden to step aside for another candidate to face Donald Trump.

He called on his ally to "pass the torch."

Schiff, a key congressional powerbroker, praised Biden but told the Los Angeles Times that he doubts that the 81-year-old incumbent can defeat Trump – a threat to "the very foundation of our democracy."

The Californian, who is expected to win a Senate seat this November, is one of the party's most influential members and a key ally in the legislature for the White House.

He served as chairman of the House intelligence panel when Democrats had the majority in 2019 and shot to nationwide prominence as lead prosecutor during then-president Trump's first impeachment trial.

In a statement to the Los Angeles Times, Schiff aired the worry gripping the party, although often in private, that questions over Biden's age and health will doom him in the November election.

Those concerns rocketed after Biden performed badly in a televised debate with Trump last month, often looking confused or unable to articulate.

Since then, Biden has repeatedly said he intends to stay in the race, arguing that he remains the best person to defeat Trump. Polls show a tight overall contest, but with Trump pulling ahead in key swing states.

Schiff called Biden "one of the most consequential presidents in our nation's history."

But he also said "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."

Biden should "secure his legacy" by stepping aside, he added. (AFP)