President Biden acknowledged on Tuesday that he “fell asleep on the stage” during his disastrous debate last week, blaming his performance on the fact that he had traveled “around the world a couple times” in the two weeks before the face-off with former President Donald J. Trump.
“I wasn’t very smart,” Mr. Biden, 81, told donors at a fund-raiser in Virginia. “I decided to travel around the world a couple times, I don’t know how many time zones.”
“It’s not an excuse but an explanation,” he said.
White House officials have blamed Mr. Biden’s having a cold at the time for his disjointed debate performance. Karine Jean-Pierre, the White House press secretary, repeated that explanation at a briefing for reporters on Tuesday afternoon.
But Mr. Biden offered a different reason to the donors on Tuesday night. He referred to his decision to travel to France for several days two weeks before the debate and return to the United States before heading back to Europe for the Group of 7 summit in Italy.
He decided to make that cross-Atlantic trip back and forth, Mr. Biden said, blaming himself for not having “listened to my staff,” which he implied had told him not to do that. He said the decision caused him to be tired during the debate.
Mr. Biden’s comments came as the White House struggled to respond to a chorus of anxiety within the Democratic Party about whether the president is capable of mounting a winning campaign against Mr. Trump in November.
For more than a year, Mr. Biden and his aides have repeatedly denied that the president’s age has affected his ability to perform his duties. They have repeatedly criticized journalists who raised the issue that large majorities of voters say they believe Mr. Biden is too old to be president.
But the debate, which was watched by about 51 million people, raised serious doubts among voters and many Democratic activists. Democratic lawmakers also expressed concerns, with the first sitting member of Congress, Representative Lloyd Doggett of Texas, calling on Mr. Biden to drop out of the race on Tuesday.
The president and his campaign have refused to even consider doing that. They say Mr. Biden remains determined to stay in the race and to defeat Mr. Trump, who he says is a threat to democracy in America.