But Mr. Biden’s performances have been inconsistent over the course of two vice-presidential debates and through this season of nearly a dozen Democratic primary debates. Even his supporters say that, at 77, his voice is less firm and that he appears less energetic and passionate than just eight years ago. Mr. Trump has highlighted some of those moments to try to raise doubts about his opponent’s mental acuity.
The big question is which version of Mr. Biden will be on the stage Tuesday night. Typically, the first debate carries the most weight, and that might be particularly the case in a year in which so many people are casting early votes by mail and in person.
Will it be the vigorous, engaged former senator and vice president who has a firm grasp over both his own record and that of his opponent, adept at delivering the punch and nimble enough to adjust to this entirely unconventional opponent? Or will it be the Mr. Biden who sometimes seemed distracted during the early Democratic debates, sparking to life at some times but at others adrift in the tumult of a crowded stage?
A defining image from the Democratic debates was the summer 2019 exchange in which Kamala Harris, who is now Mr. Biden’s running mate, attacked him for opposing school busing earlier in his Senate career. Mr. Biden looked flustered and confused, finally saying: “I agree that everybody wants … Anyway, my time is up. I’m sorry.”
‘You’d Be in Jail’
Moments after the start of the first 2016 general election debate, Mr. Trump’s strategy was clear. Again and again — amid head-spinning diversions, theatrical asides, breathtakingly brutal attacks, outright fabrications and displays of showmanship — he returned to the themes that had defined his candidacy.
Mrs. Clinton was, by his repeated telling, a tired soldier of Washington’s old guard, which had brought the nation job-killing trade deals, a dangerous nuclear arms deal with Iran, out-of-control immigration and Obamacare. (“Obamacare is a disaster,” he said at their second debate. “You know it. We know it.”)
As unorthodox a politician as he was, Mr. Trump was executing a traditional debate strategy. But over the course of those three debates, he would go places presidential candidates rarely went, showing no hesitation about launching any attack.