Biden’s advisers planned a strategy for the debate for months. Then everything fell apart.

This story is based on conversations with eight people involved in or briefed on the president’s debate preparation, all of whom spoke on condition of anonymity to describe private meetings. The Biden campaign declined to comment.

For a full week, the president holed up at Camp David with more than a dozen advisers to prepare for Thursday’s presidential debate with former President Donald Trump. He rehearsed answers, met with political advisers and participated in mock debates, with his personal lawyer, Bob Bauer, playing the role of Trump.

He had practiced answers for every topic he was asked about Thursday, including the last one about his age.

So his aides were baffled by his performance. Many felt they had never seen him break down so dramatically. After all, Biden was a veteran of numerous debates — as a senator, a vice presidential candidate and a presidential candidate. And they didn’t understand why he gave a completely different answer to the age question than the one they spent more than a week perfecting.

The president not only struggled with his words, but seemed to lose concentration and often failed to finish his sentences. Her voice sounded hoarse and weak, and when the debate concluded, first lady Jill Biden appeared to help her husband down the stairs.

His performance sent shockwaves through the Democratic Party, prompting some Democrats to call for him to step aside. In the 48 hours after the debate, Biden campaign officials sought to reassure supporters and donors, blaming the debate on “just a bad night” and promising that the president would remain in the race. They argued that the president should be judged by his three and a half years in office, not the 90 minutes he was on stage.

But with another debate scheduled for September — a Biden campaign spokesman said the president would not withdraw from that one — advisers and allies are scrutinizing the president’s preparation for last week’s debate to determine whether they missed signs of what would unfold in CNN’s studio in Atlanta.

Over the years, Biden’s aides have developed a proven formula for preparing him for debates, a process overseen by Ron Klain, his first White House chief of staff and a longtime debate guru for Democratic presidential candidates. Early in the process, Biden often meets with Klain individually or with a small group of aides to practice answers, sometimes writing his favorite responses on index cards — his way, aides say, of clarifying his thinking.

Aides working in specific subject areas, such as national security or the economy, will join in preparing for sessions focused on those topics. Finally, Biden will participate in mock debates, designed to mimic the real event as closely as possible.

At Camp David, Biden participated in several such mock sessions, held in a movie theater and an airplane hangar set up to resemble CNN’s studio. They were held at different times of the day, including at night, when Biden sometimes appears to falter and when the debate was to take place, officials said.

Some Biden officials speculated that the president was overprepared after days of lengthy preparation sessions and got into his own head. Others lamented that too many aides were part of the preparations, noting that the White House distributed a list of 18 officials who accompanied the president to Camp David — and it didn’t even include everyone who participated.

Not all of these people were in the room with Biden at all times, people familiar with the preparations said, but they still noted that there is a risk of having too many opinions, which can be contradictory and confusing.

As the debate began Thursday, Biden’s top advisers, gathered in a waiting room at the CNN studios, knew immediately that the president had gotten off to a rocky start, stumbling over answers about the economy and ending his remarks on the national debt with a gaffe: “We finally beat Medicare.” Trump seized on the gaffe.

Early in the debate, Biden officials began telling reporters that the president had a cold and a sore throat in an effort to explain why his voice sounded weak and hoarse.

“We were asked about his hoarse voice,” White House press secretary Karine Jean-Pierre told reporters aboard Air Force One on Friday. “We shared that he had a cold, we shared that he tested negative for Covid, and then we moved on. That’s it. It was because of his voice while he was speaking during the debate, obviously.”

Still, at a lively campaign rally the next day, Biden’s voice showed no sign of the hoarseness of the night before.

Biden’s advisers and allies had been concerned for months about whether he should participate in the traditional presidential debates. Some argued strenuously that he should not participate, worried that he would not be up to the task and that Trump would dominate the debate with his shouting and bluster.

Others thought that if Biden did not participate in a debate, he would be weak and predicted that he would do well, as demonstrated by the powerful presentation of his State of the Union address, when he engaged in an exchange of comments with the Republican legislators who met him. they booed.

Biden’s campaign eventually agreed to hold two debates, insisting that the events be held on his terms: There would be no studio audience and each candidate’s microphone would be muted when it was not his turn to speak. Some allies later speculated that those rules might have helped Trump by curbing his impulse to interrupt his opponent and draw in the crowd — actions that seemed to turn off viewers in previous debates.

And Biden’s advisers demanded an unusually early date for the first meeting, hoping that a strong performance would boost the president’s campaign and calculating that it would give Biden time to recover if he failed. Instead of boosting the campaign, the event has sparked a flurry of new calls for her to reconsider his candidacy.

On Thursday night, after Biden left the debate studio, he stopped by a Waffle House and told reporters that he thought he “did good.” But in the hours that followed, the campaign scrambled to do damage control. Officials worked to reassure donors, allies warned against exaggerating the effect of a bad night, and Biden himself debuted a new line addressing his age.

“I don’t walk as easily as I used to,” he said Friday at a boisterous rally in Raleigh, North Carolina. “I don’t speak as fluently as I used to. I don’t debate as well as I used to. But I know what I know: I can tell the truth.”

Later Friday, at a fundraiser, the first lady told donors that her husband had admitted that something went wrong.

“After last night’s debate, he said, ‘You know, Jill, I don’t know what happened. I didn’t feel that good,’” she recounted. “And I said, ‘Look, Joe, we’re not going to let 90 minutes define the four years you’ve been president.’”

At the end of his three-day tour, even the president publicly acknowledged that the night did not go as he expected.

“I understand the concern after the debate. I get it, I didn’t have a great night,” she said at a fundraising event Saturday night in Red Bank, New Jersey. “But I’m going to fight harder.”

Michael Scherer in Washington contributed to this report.