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.

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.