By the time the presidential debate was over, President Biden looked like some athlete in a contact sport who waited too long to retire. Slack-jawed, hoarse, dazed, gazing vacantly into space, desperately trying to complete a thought, a sentence, or distinguish between billions, millions, or trillions, Mr. Biden looked like he was barely going to make it safely to bed tonight, let alone to the end of his presidency. Asking America to trust him to lead the country for another four years seemed preposterous.