Jill Biden Could Make or Break Biden’s Campaign. She Says She’s All In.

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

Whatever was going to happen next in Mr. Biden’s last presidential race, after perhaps the worst moment of his long political life, was always going to come down to her. His wife of 47 years had entered his life all those decades ago, reluctant to get into 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 after a disastrous 90 minutes onstage.

The first lady’s message to him was clear: They’d been counted out before, she was all in, and he — they — would stay in the race. Her thinking, according to people close to her, was that it was a bad night. And bad nights end.

“To say they’ve been in foxholes together doesn’t even begin to explain their bond,” said Elizabeth Alexander, the first lady’s communications director, who has been with Mr. Biden since his Senate days.

So Dr. Biden spent the 24 hours after the debate putting her decades as a political spouse to the test, projecting 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 along with the cheering supporters, there are legions of people suddenly accusing her of forcing an old man to put one weary foot in front of the other.

If Mr. Biden were to seriously consider departing the race, allowing a younger candidate to replace him, the first lady would be the most important figure — other than the president himself — in reaching that decision.