On his last day as president on Jan. 20, 2021, Donald J. 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.

Mr. 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 fund-raisers assured donors they were done with him. On conference calls, House Republican leaders contemplated a “post-Trump” G.O.P.

Today, three years after Jan. 6 and more than a week before the Iowa caucuses, Mr. Trump has almost entirely subjugated the elected class of the Republican Party. As of this week, every member of the House Republican leadership is formally backing his campaign to recapture the White House.

Mr. Trump has obsessed over his scorecard of endorsers, according to more than half a dozen Trump advisers and people in regular contact with him, most of whom insisted on anonymity to describe private conversations.