Washington DC - Democrats shocked by Joe Biden's dismal debate performance are urging the president to be transparent about his mental fitness as he faced the first call from his own side to drop out of the election.

Many supporters have expressed growing doubts about the 81-year-old's candidacy after last week's disastrous showdown with Donald Trump, when Biden repeatedly stumbled over his words and lost his train of thought.

Congressman Lloyd Doggett became the first Democratic lawmaker to publicly call on Biden to make way for another candidate, saying he was hopeful the president would "make the painful and difficult decision to withdraw."

Former House speaker Nancy Pelosi admitted it was "legitimate" to ask whether Biden's debate disaster was indicative of a deeper problem rather than a one-off.

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Rhode Island Senator Sheldon Whitehouse told WPRI-TV he was "pretty horrified" by the president's performance during the 90-minute CNN match-up, watched by more than 50 million Americans.

And Jared Golden, a vulnerable Democrat in a conservative-leaning House district, raised eyebrows with an op-ed in his local paper in Maine in which he said Biden's poor showing "was not a surprise."

"The outcome of this election has been clear to me for months: while I don't plan to vote for him, Donald Trump is going to win," he wrote.

Biden has not given a live interview since the debacle, but will be interviewed by ABC on Friday, with the first clips released later that day.

On Tuesday, he blamed exhaustion from international travel for his debate flop.

Speaking at a fundraiser, Biden said he "wasn't very smart" for "traveling around the world a couple times... shortly before the debate."

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"I didn't listen to my staff," he said. "And then I almost fell asleep on stage."

Biden traveled to France from June 5 to 9 to commemorate the 1944 Allied landings, headed back to the US, and then flew to Italy for a G7 summit followed immediately by a visit to California.

He then returned home and went to a presidential retreat for several days of rest and preparation for the debate.

White House Press Secretary Karine Jean-Pierre on Tuesday admitted the debate was "a bad night" but added that Biden "knows how to come back" from adversity.

She dismissed questions about him needing a cognitive test, and said the president would hold a press conference during a NATO summit in Washington next week.

The White House said Biden would meet with Democratic governors on Wednesday evening.

One of them, J.B. Pritzker of Illinois, explained that the governors had asked for the opportunity to hear from the president directly.

"Right now, Joe Biden is our nominee, and I'm 100% on board with supporting him as our nominee, unless he makes some other decision, and then I think we're all going to be discussing what's the best way forward," he told CNN