Questions about Mr. Biden’s fitness have nonetheless taken a toll on his public standing. In a June survey by Harvard’s Center for American Political Studies and the Harris Poll, 64 percent of voters believed he was showing that he is too old to be president, including 60 percent of respondents 65 or older.
Mr. Biden’s public appearances have fueled that perception. His speeches can be flat and listless. He sometimes loses his train of thought, has trouble summoning names or appears momentarily confused. More than once, he has promoted Vice President Kamala Harris, calling her “President Harris.” Mr. Biden, who overcame a childhood stutter, stumbles over words like “kleptocracy.” He has said Iranian when he meant Ukrainian and several times called Senator Mark Warner, Democrat of Virginia, “John,” confusing him with the late Republican senator of that name from Virginia.
Republicans and conservative media gleefully highlight such moments, posting viral videos, sometimes exaggerated or distorted to make Mr. Biden look even worse. But the White House has had to walk back some of his ad-libbed comments, such as when he vowed a military response if China attacks Taiwan or declared that President Vladimir V. Putin “cannot remain in power” in Russia.
Mr. Biden was famously prone to gaffes even as a younger man, and aides point to his marathon meetings with families of mass shooting victims or his working the rope line during a trip to Cleveland this past week as evidence of stamina.
Mike Donilon, a senior adviser who began working for Mr. Biden some 40 years ago, said he did not see much change. “On the way back from long trips when the staff is wiped out, he’ll want to spend four hours planning for how we hit the ground running on domestic policy, when all much younger staff want to do is sleep,” he said.
Mr. Biden is not the first president to confront questions of age. The issue came up repeatedly under President Donald J. Trump, who is four years younger. Mr. Trump’s diminished vocabulary, tendency to meander, sometimes incoherent remarks, light office schedule and struggles to process information led critics to conclude that he was in decline.