But the president’s performance cannot be written off as a bad night or blamed on a supposed cold, because it affirmed concerns that have been mounting for months or even years. Even when Mr. Biden tried to lay out his policy proposals, he stumbled. It cannot be outweighed by other public appearances because he has limited and carefully controlled his public appearances.

It should be remembered that Mr. Biden challenged Mr. Trump to this verbal duel. He set the rules, and he insisted on a date months earlier than any previous general election debate. He understood that he needed to address longstanding public concerns about his mental acuity and that he needed to do so as soon as possible.

The truth Mr. Biden needs to confront now is that he failed his own test.