President Biden struggled through his first debate of the 2024 campaign against Donald J. Trump, meandering and mumbling through answers as the former president pressed his case for a second term with limited resistance from his rival.

They disagreed on abortion, inflation, climate change, foreign affairs and immigration. But the sharpest contrast was in their presentation.

Mr. Trump was confident and forceful, even as he let loose a stream of misleading attacks and falsehoods. Mr. Biden spoke with a hoarse and halting voice, closing his eyes occasionally to gather thoughts that sometimes couldn’t be corralled. Democratic anxiety rose by the minute. About halfway through, people close to Mr. Biden put out word that he had a cold.

Mr. Trump relentlessly hammered Mr. Biden on areas of vulnerability, sending exaggerations and embellishments — he was the “greatest” and his opponent the “worst” — flying unchecked through the audience-free CNN studio in Atlanta.

Mr. Biden’s allies desperately hoped he could turn in a commanding performance to calm voters’ persistent concerns about his age.