Last January, on the night of John McCain’s back-from-the-dead victory in the New Hampshire primary, I asked a longtime adviser how McCain had survived the collapse of his campaign a few months earlier. “You know, in the darkest days, I think there were a lot of us who just respected him so much we just wanted to band together to make sure to restore his dignity,” the adviser told me. “But the chance of this actually happening was pretty remote.” And yet it did happen. After McCain won New Hampshire, he kept on winning until there he was, onstage last...