President Joe Biden vigorously defended his fitness for office in a high-stakes press conference Thursday evening, feeling several questions about his age and calls for him to step aside.

“I think I’m the most qualified person to run for president,” he said, adding that he believes he is the best candidate to beat former President Donald Trump in November. “I beat him once, and I will beat him again.”

Early in the press conference, in a moment that was immediately seized on by his opponent…