From the very first question, President Biden’s voice was a muted rasp.
“You’ve got to take a look at what I was left when I became president” — cough — “what Mr. Trump left me.”
Mr. Biden, the 46th president, entered Thursday’s debate with the 45th, Donald J. Trump, needing to calm concerns about his own age and mental acuity. Instead, Mr. Biden’s incoherent performance inflamed those fears, raising questions from the start about whether he would be able to carry on as the Democratic nominee.
Any talk of Mr. Biden’s relying on performance-enhancing drugs to survive the debate seemed quaint by the time he opened his mouth on the debate stage and suffered what seemed like a prime-time meltdown.
Even Mr. Trump looked almost taken aback as his adversary stumbled and struggled to get his words out. “I really don’t know what he said at the end of that sentence,” Mr. Trump said after Mr. Biden answered a question about border security not 10 minutes into their debate. “I don’t think he knew, either.”
It was all the more startling considering Mr. Biden had cleared his calendar and holed up at Camp David for days to prepare. (White House officials said that Mr. Biden had a cold.)
Mr. Trump’s message was often factually incorrect, but it was one communicated clearly, fiercely and impatiently. Mr. Biden, in contrast, stood slack-jawed, his eyes darting back and forth, while his opponent spoke.
Early in the debate, Mr. Biden briefly seemed to go blank, stumbling in a syntax-free way to the end of a long point he was trying to make about health care. He finally seemed to give up, saying, “Look. If — we finally beat Medicare.”
The strangeness of the moment did not prevent Mr. Trump from pouncing: “Well, he’s right. He did beat Medicare — he beat it to death.”
Mr. Biden tried some of the lines he leaned on in earlier debates — “I’ve never heard so much malarkey in my life,” he said at one point — but it underscored only how different he sounded now, even compared with just four years ago.
Gone was the confident ear-to-ear grin in the split screen.
At times, Mr. Biden seemed to want to go for the jugular, as when he invoked the hush-money payments Mr. Trump made to a porn star that resulted in 34 felony convictions. But the attack came out garbled, ending rather lamely as Mr. Biden described his opponent as having “the morals of an alley cat.”
Mr. Biden repeatedly interrupted himself and trailed off mid-answer. While discussing abortion, arguably his strongest issue, he interjected a mention of immigration and crime, his weakest. “A young woman who just was murdered, and he went to the funeral. The idea that she was murdered by a — by an immigrant coming into — they talk about that,” he said.
Later, he seemed to come up with a new way to define the three trimesters of pregnancy.
“I supported Roe v. Wade, which had three trimesters,” he said. “First time is between a woman and a doctor. Second time is between the doctor and an extreme situation. A third time is between the doctor. I mean, it would be between the woman and the state.”
At times, such as in an exchange about Mr. Trump’s reported comments about veterans, Mr. Biden seemed enraged and yet not quite able to express his anger. At other points, as when he talked about climate change and historically Black colleges and universities, he seemed almost out of breath.
Talking about the war in Ukraine, Mr. Biden seemed to confuse Mr. Trump with Mr. Putin, saying: “If you take a look at what Trump did in Ukraine, he, this guy told Ukraine, told Trump, do whatever you want, do whatever you want, and that’s exactly what Trump did. Putin encouraged him, do whatever you want. And he went in.”
Toward the end of the debate, Mr. Biden appeared to perk up while talking about affordable child care. And he landed a blow when he called Mr. Trump a “whiner” who “snapped” when he lost the election in 2020. But by then, the hour was late.