President Joe Biden has blamed his performance during last week's debate on jet lag after two overseas trips earlier in June.

The 81-year-old has faced mounting questions about his 2024 re-election bid after last week's shaky debate performance.

It comes as one House of Representatives Democrat, Lloyd Doggett, has publicly called on him to withdraw from the race.

Speaking at a campaign event in McLean, Virginia, President Biden conceded the debate against former President Donald Trump, his Republican rival, did not go well.

Speaking without the aid of a teleprompter, President Biden said: "I didn’t have my best night, but the fact is that you know, I wasn’t very smart. I decided to travel around the world a couple times, going through around 100 time zones...before...the debate.

"Didn’t listen to my staff and came back and nearly fell asleep on stage...That's no excuse but it is an explanation."

Biden had travelled to France and Italy during two separate trips in the space of two weeks last month, flying overnight from the Group of Seven summit in Bari, Italy, to appear at a fundraiser with former President Barack Obama in Los Angeles on June 15 before returning to Washington the following day.

He then spent six days at Camp David preparing for the June 27 debate. White House officials have blamed Biden's halting performance during the debate on a cold. Biden did not mention being sick during the fundraiser.

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A new Ipsos poll showed that one in three Democrats think Biden should end his re-election bid following the debate. However, it also found no prominent elected Democrat does any better than Biden in a hypothetical matchup against Trump.

The poll found that both Trump and Biden maintain the support of 40 per cent of registered voters, suggesting that Biden has not lost ground since the debate.

Vice President Kamala Harris, asked in a CBS News interview about Representative Doggett's demand, said: "Look, Joe Biden is our nominee. We beat Trump once and we're gonna beat him. Period."

US Representative Jim Clyburn, the Democrat often credited with securing Biden's nomination in the 2020 presidential race, told reporters he would support Vice President Kamala Harris to be the Democratic nominee if Biden stepped aside.

Representative Jared Golden, a Maine Democrat, predicted on Tuesday that Trump would win the election, as did Washington state Democratic Rep. Marie Gluesenkamp Perez.

Elsewhere, Donald Trump's sentencing for his conviction on criminal charges stemming from hush money paid to adult film star Stormy Daniels was delayed until September 18.

Justice Juan Merchan pushed back the sentencing date so he could weigh the former US president's argument he should have been immune from prosecution under the Supreme Court ruling that presidents cannot be criminally prosecuted for official acts.

The new timeline means Justice Merchan could decide the Republican presidential candidate's punishment.