Biden has faced mounting questions about his 2024 re-election 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, Biden, 81, admitted last Thursday's debate against former US 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 times, going through around 100 time zones ... before ... the debate.
Joe Biden has conceded the presidential TV debate against Donald Trump was not his best night. (AP PHOTO)
"Didn't listen to my staff and came back and nearly fell asleep on stage," he said. "That's no excuse but it is an explanation."
Biden travelled to France and Italy during two separate trips in the space of two weeks last month, flying overnight from the G7 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.
Former House Speaker Nancy Pelosi has admitted concerns about Joe Biden's physical abilities. (AP PHOTO)
Former House Speaker Nancy Pelosi told MSNBC that she believes "it is a legitimate question" whether Biden's performance was just "an episode or is this a condition."
"When people ask that question, it's legitimate - of both candidates," Pelosi said.
But she did not go so far as to ask Biden to step aside.
Pelosi said she has heard "mixed" opinions about whether Biden is up for the gruelling campaign ahead.
A new Reuters/Ipsos poll that closed on Tuesday showed that one in three Democrats think Biden should end his re-election bid following the debate, but no prominent elected Democrat does any better than Biden in a hypothetical match-up against Trump.
The two-day poll found that both Trump, 78, and Biden, 81, maintain the support of 40 per cent of registered voters, suggesting that Biden has not lost ground since the debate. Election Day is November 5.
with AP
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