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.
Mr Biden’s remarks come amid intra-party panic over his mental fitness, and after a congressman from Texas became the first sitting Democratic lawmaker to call for him to step aside following his debate.
“I am hopeful that he will make the painful and difficult to decision to withdraw,” Rep Lloyd Doggett said in a statement on Tuesday.
President Biden appeared to struggle through some responses during a debate with former President Donald Trump last Thursday.
On Tuesday, he spoke at a private fundraiser in Virginia.
“It’s not an excuse but an explanation,” he said, 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 returned from Los Angeles on 16 June. The week before he had attended D-Day anniversary events in France.
His age has been a long-simmering issue this election, with voters in multiple polls saying they think he is too old to be effective.
Mr Biden is currently the Democratic Party’s presumptive nominee for the White House. He has vowed to stay in the race despite the debate performance.
In his statement, Rep Doggett, 77, said the debate solidified his decision to urge Mr Biden to step aside.
“Instead of reassuring voters, the President failed to effectively defend his many accomplishments and expose Trump’s many lies,” said Rep Doggett, who was sworn in in 1995 and is running for reelection.
He said too much is at stake to risk the president losing to Trump over fears about his age.
“While much of his work has been transformational, he pledged to be transitional,” the congressman said of Mr Biden.
“He has the opportunity to encourage a new generation of leaders from whom a nominee can be chosen to unite our country through an open, democratic process.”
“My decision to make these strong reservations public is not done lightly nor does it in any way diminish my respect for all that President Biden has achieved,” Rep Doggett said.
Mr Biden will give a primetime interview to ABC News’ George Stephanopoulos on Friday, his first since the debate.
Some prominent Democratic lawmakers voiced their concerns about Mr Biden’s age and stamina this week, but none until Rep Doggett has called for him to move aside as a candidate.
Other top Democrats have acknowledged fears about Mr Biden’s ability to win, but emphasised that the choice to leave the race is the president’s alone.
Several have flocked to liberal-leaning network MSNBC to defend him.
“It’s going to be up to Joe Biden” to do what he thinks is best, former US House Speaker Nancy Pelosi told MSNBC on Tuesday.
One of President Biden’s most important backers, Congressman Jim Clyburn of South Carolina, said he would support Vice-President Kamala Harris as the party’s nominee if Mr Biden stepped down.
But he told the network: “I want this ticket to continue to be Biden-Harris.”
Congressman Jamie Raskin, a Democrat from Maryland, told MSNBC this weekend that the debate created a “difficult situation”.
He acknowledged that there were “very honest and serious and rigorous conversations taking place at every level of our party.”
But he added: “Regardless of what President Biden decides, our party is going to be unified and our party also needs him at the very centre of our deliberations in our campaign.”
The Biden campaign did not immediately respond to a request for comment.