Last week, when the Democratic National Convention, in its virtual state, nominated Joseph R. Biden as its Presidential candidate, Donald Trump decided to let his supporters in on a secret. “Joe, look, he doesn’t know where he is,” the President said, leaning toward a microphone that had been set up in front of Air Force One, in Yuma, Arizona, where he had come to accept the endorsement of the union representing border-control agents. Biden, he said, wouldn’t be able to resist the commands of “his new boss,” Bernie Sanders. In Yuma and elsewhere, in speeches that grew darker and stranger as the week progressed, Trump pounded on the same theme: Biden was “a puppet,” a “Trojan horse for socialism,” the smiling, cognitively unsound prop of a left-wing mob intent on…