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President Joe Biden has blamed his poor debate performance last week on jet lag, telling reporters that he "wasn't very smart" for "travelling around the world a couple of times" before the debate.
"I didn’t listen to my staff... and then I nearly fell asleep on stage,” he said.
Mr Biden, 81, last returned from travel on 15 June, nearly two weeks ahead of the 27 June debate.
The president's remarks come amid intra-party panic ahead of November's election over his mental fitness, and after a congressman from Texas became the first sitting Democratic lawmaker to call for him to step aside following the debate.
"I am hopeful that he will make the painful and difficult decision to withdraw," Rep Lloyd Doggett said in a statement on Tuesday.
Mr Biden appeared to struggle through some responses during a debate with former President Donald Trump last Thursday.
"It’s not an excuse but an explanation,” he said at a private fundraiser in Virginia on Tuesday evening, referring to his travel.
He also apologised for his performance and said it was "critical" that he win re-election, according to ABC News.
Mr Biden made two separate trips to Europe within two weeks last month.
On 15 June, he appeared at a fundraiser alongside former President Barack Obama after an overnight trip from Italy. He returned to Washington DC the following day.
White House officials have previously said he was battling a cold on the day of the debate.
The president did not mention any illness in his remarks on Tuesday. A spokeswoman for the White House said earlier in the day that he was not taking any cold medication during the debate.
Mr Biden also spent six days at Camp David, the presidential retreat outside Washington DC, preparing for his debate against Donald Trump.
The New York Times, citing an unnamed source familiar with Mr Biden's schedule, reported on Tuesday that his days began at 11:00 each morning and that he was given time each day to nap.
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