On the evening of June 27, 2024, millions of viewers watched with alarm and confusion as President Joseph R. Biden Jr., appearing listless and lost, made an ultimately campaign-ending appearance in his only presidential debate against Donald J. Trump.

Among the viewers who came away frightened: his wife, Jill Biden.

“I don’t know what happened,” the former first lady said in an interview with “CBS News Sunday Morning.” “As I watched it, I thought, ‘Oh, my God, he’s having a stroke.’ And it scared me to death.”

In a 30-second snippet of the interview, which is scheduled to air in full this weekend, she said that she had never seen her husband have a meltdown like the one she saw when he took the debate stage in Atlanta. Next week, she is releasing “View From the East Wing,” a memoir of her time as first lady.

Mr. Biden, who had made back-to-back trips to Europe in the weeks leading up to the debate, has said that he was recovering from an illness that evening. When Mr. Biden was preparing at Camp David, his team had built naps into his schedule. Despite the extra rest, a raspy-sounding Mr. Biden mumbled and wandered through his appearance as Mr. Trump hammered him on immigration and foreign policy.

“I was frightened,” Dr. Biden told CBS of what she saw that night, “because I had never, ever seen Joe like that. Before or since. Never.”

Her memoir, according to the publisher, Simon & Schuster, promises to bring readers “behind the scenes” of her life as first lady, including what it was like to witness “the abrupt end of her husband’s bid for re-election.”

But on the night of the debate and for weeks afterward, she was not just a witness but a driving force behind Mr. Biden’s decision to keep fighting to salvage his campaign for re-election. She comforted the shaken president in the minutes after the debate ended, and told him that she was still all in.

“You did such a great job, you answered every question,” she told her husband when they joined supporters after the debate, “you knew all the facts.”

In the weeks following the debate, Dr. Biden and other members of Mr. Biden’s family, including his son, Hunter, were supportive and encouraged him to keep fighting.

But as Mr. Biden was recovering from the coronavirus in late July, his wife began to stress to him that he needed to make a decision. He ended his campaign and endorsed Kamala Harris, his vice president.

As first lady, Dr. Biden was a protective force around Mr. Biden. She gained a reputation for bluntly pointing out when she felt that Mr. Biden’s advisers had made mistakes when it came to her husband.

“I don’t think it’s a bad thing,” she told this reporter when asked about that reputation during an interview. “Do you?”