Jill Biden can make or break Biden’s campaign. She says she’s all in.

President Biden knew immediately after he left the stage in Atlanta on Thursday evening that the debate had gone wrong. In those first stricken moments after a raspy, rambling and sometimes incoherent performance, he turned to his wife, Jill Biden.

Whatever happened in Mr. Biden’s final presidential race, after perhaps the worst moment of his long political life, it would always come down to her. His wife of 47 years had come into his life all those decades ago, reluctant to enter politics but fully embracing his dreams and his belief that he would one day reach the White House.

Now her 81-year-old husband looked at her, stung and staggering.

The first lady’s message to him was clear: They had been counted out before, she was all in, and he — they — were going to stay in the race. According to people close to her, she thought it was a bad night. And bad nights end.

“If you say they’ve been in the trenches together, that doesn’t even begin to explain their bond,” said Elizabeth Alexander, the first lady’s communications director, who has been with Biden since his time in the Senate.

So Dr. Biden spent the 24 hours after the debate putting her decades as a political wife to the test, exuding confidence and normalcy while effusively praising her husband. But like the president, she is an intuitive political messenger who can sense the mood of a crowd. She knows that in addition to the cheering supporters, there are also legions of people suddenly accusing her of forcing an old man to put one tired foot in front of the other.

If Biden were to seriously consider withdrawing from the race and letting a younger candidate take his place, the first lady would be the most important figure — besides the president himself — in making that decision.

“Jill is the last and most important voice. She knows him and loves him with passion. She also knows everything about him. Most of the big decisions are ultimately made with Valerie and Jill,” said John Morgan, one of Biden’s biggest donors, referring to the president’s younger sister, who has managed nearly all of his political campaigns.

As the Democratic Party’s major donors connected Friday, whether by text, by phone or in person, one of the most direct questions they asked each other was whether any of them knew how to arrange a meeting or conversation with the first lady could get.

After nearly half a century in politics, the Bidens see themselves as people who can stay in the game for the long haul. And right now, neither wants the story of the president’s long political career — one marked by tragedy, resilience and relentless ambition — to end on a stage in Atlanta, facing off against former President Donald J. Trump, a man they both despise.

“He wants to win and she wants it for him and for the country,” Ms. Alexander said. “She is his biggest supporter and champion because she believes in him and she fears for the future of our country if things go the other way.”

On Friday, in front of her supporters, the first lady embraced the views of Democratic Party leaders, including Vice President Kamala Harris, that Biden’s poor performance does not undo years of successful legislation.

“As Joe said earlier today, he is not a young man,” Dr. Biden told a group of donors gathered in Manhattan on Friday afternoon, her third stop since leaving Atlanta. “After last night’s debate he said, ‘You know, Jill, I don’t know what happened. I didn’t feel great.’ I said, ‘Look, Joe, we’re not going to let 90 minutes define the four years you’ve been president.’”

Dr. Biden understood that the debate night had been a serious misstep. The president should have walked into the debate hall and addressed concerns about his age. Instead, he walked onto the stage after six days of preparations and mock debates at Camp David with little to show for it other than a hoarse voice. (The White House said he had a cold.)

She listened as Mr. Trump mocked him. “I really don’t know what he said at the end of that sentence,” Mr. Trump said when Mr. Biden gaffes in an answer about immigration. “I don’t think he knows what he said either.”

She listened as former President attacked Hunter Biden, Mr. Biden’s son whom she had raised since childhood and assisted during a recent trial on gun charges, sitting in the front row of the courtroom as the worst moments of his addiction for the world were told. to see.

And she watched her husband, wide-eyed and open-mouthed, as Mr. Trump continued, taking what was happening angrily but barely able to strike back.

Then Dr. Biden held the hand of the president, who carefully walked down the stairs. The moment quickly went viral. During a campaign-organized viewing party that the Bidens attended shortly after the debate, she praised her husband for his performance. But critics elsewhere saw her virtually patting him on the head for simply surviving the debate.

“You answered every question, you knew all the facts,” she said. “And what has Trump done?”

“Lie!” the crowd shouted.

Suddenly, a first lady who had avoided major controversy for the past three and a half years found herself in the crosshairs of people who believe she tried to hide her diminished abilities.

“What Jill Biden and the Biden campaign did to Joe Biden tonight — rolling him onto the stage to engage in a battle of wits while he was unarmed — is elder abuse, pure and simple,” wrote Rep. Harriet M .Hageman, Republican from Wyoming, in a social media post.

The Drudge Report, a prominent conservative-leaning website whose author, Matt Drudge, loathes Trump, on Friday published an unflattering photo of the Bidens with the headline “CRUEL JILL CLINGS TO POWER.”

The first lady and her advisers have long noticed similar claims on conservative websites, and are aware that they are leaking into the mainstream. Ms. Alexander said that Dr. Biden views her “amorphous” role as “an act of service, rather than some mythical power grab invented by the dark corners of the internet.”

She added that the first lady sometimes felt hampered by the demands of the role, a role full of expectations and hidden stumbling blocks.

“You have to be supportive, but not so supportive that your motives are questioned,” Ms. Alexander said, placing much of the blame on the internet, bots and a right-wing machine that fuels “every conspiracy.”

Advisers to the president and the first lady have downplayed the idea that she has the ability to unilaterally pull the plug on the president’s re-election campaign and clear the way for a new candidate with four months to go. They acknowledge her unique influence and power in his life, but they say Mr. Biden is in control of his own campaign.

“There’s too much on Jill’s shoulders,” said one of Biden’s top advisers, who spoke on condition of anonymity to discuss family dynamics. But as long as Biden wants to run, she will support him, advisers say.

“When Joe gets knocked down, Joe gets back up,” she told donors in New York. “And that’s what we’re doing today.”