U.S. President Joe Biden acknowledged on Tuesday that his performance during last Thursday's presidential debate wasn't his best but blamed it on jet lag after two overseas trips earlier in June.

Biden has faced mounting questions about his 2024 reelection bid after last week's shaky debate performance, with one House of Representatives fellow Democrat on Tuesday publicly calling on him to withdraw from the race.

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

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

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

Biden traveled 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 Tuesday's fundraiser.

A new Reuters/Ipsos poll that closed on Tuesday showed that one in three Democrats think Biden should end his reelection bid following the debate, but no prominent elected Democrat does any better than Biden in a hypothetical matchup against Trump.

The two-day poll found that both Trump, 78, and Biden, 81, maintain the support of 40% of registered voters, suggesting that Biden has not lost ground since the debate. Election Day is Nov. 5.