On his last day as president on Jan. 20, 2021, Donald Trump stood in a snapping wind and waved goodbye to relatives and supporters before he took his final flight on Air Force One back to Mar-a-Lago. No elected Republican of any stature showed up at Joint Base Andrews for the bleak farewell. Trump, at that moment, was a pariah among Republican elites. The party’s leaders in the House and Senate, Kevin McCarthy and Mitch McConnell, blamed him for the Capitol siege. Party fundraisers assured donors they were done with him. On conference calls, House Republican leaders contemplated a “post-Trump”...