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At 79, Biden is testing the limits of age and the presidency

However, questions about Mr. Biden’s fitness have affected his public standing. In a June survey by the Harvard Center for American Policy Studies and the Harris Poll, 64 percent of voters thought he was showing he was too old to be president, including 60 percent of those 65 or older.

Mr. Biden’s public appearances have fueled that perception. His speeches can be flat and apathetic. Sometimes he loses his train of thought, has trouble calling names, or seems momentarily confused. More than once, he promoted Vice President Kamala Harris, calling her “President Harris.” Mr Biden, who overcame a childhood stutter, tripped over words like “kleptocracy”. He said Iranian when he meant Ukrainian and called Sen. Mark Warner, D-Va., “John” several times, confusing him with the late Republican Virginia senator of that name.

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 campaign comments, such as when he promised a military response if China attacked Taiwan or said President Vladimir Putin “cannot stay in power” in Russia.

Mr. Biden was known for his propensity for gaffes even as a younger man, and aides point to his marathon meetings with families of mass shooting victims or his tightrope work during a trip to Cleveland last week as evidence of resilience.

Mike Donilon, a senior adviser who started working for Mr. Biden about 40 years ago, said he doesn’t see a change. “Coming back from long trips, when the staff is wiped out, he’ll want to spend four hours planning how to proceed with domestic politics when all the much younger staff want to do is sleep,” he said.

Mr. Biden is not the first president to grapple with issues of age. The issue has been raised repeatedly with President Donald J. Trump, who is four years younger. Mr. Trump’s limited vocabulary, tendency to ramble, sometimes disjointed remarks, light office schedule and difficulty processing information have led critics to conclude that he is in decline.