Mr. Trump’s capricious approach to politics was destabilizing for Republicans up and down the ballot, leaving candidates exposed to the president’s whims and grievances and the machinations of White House advisers. Rather than approaching the midterm campaign as a task of holding together a political coalition and steering it to victory, Mr. Trump focused chiefly on rewarding perceived friends — and punishing those who crossed him.
He helped drive two senators, Mr. Corker and Mr. Flake, into retirement, castigating them in humiliating terms online and driving up their unpopularity with Republicans. In gubernatorial elections, Mr. Trump helped anoint highly divisive nominees in states like Florida and Georgia — where the national G.O.P. had to burn millions staving off defeat — and Kansas, where Mr. Trump’s favored candidate, Kris Kobach, was defeated by a Democrat, Laura Kelly.
In other cases, Mr. Trump’s associates lashed out at Republican candidates who failed to prostrate themselves sufficiently. When Gov. Kim Reynolds of Iowa issued a supportive statement about a renegotiated trade deal with Canada and Mexico, a White House aide sharply rebuked a strategist for Ms. Reynolds’s campaign — because the governor did not praise Mr. Trump by name in her comment. And because of the perceived slight, Mr. Trump’s aides dropped plans for the president to headline a fund-raiser for the governor, instead insisting that the event be held for the Iowa Republican Party.
The president’s relative political inexperience also left him open to manipulation by aides and allies with agendas of their own. When a group of lawmakers and White House aides lobbied Mr. Trump to endorse Representative Raul Labrador, a hard-liner running for governor in Idaho, supporters of a competing candidate, Lt. Gov. Brad Little, sprang into action. They assembled footage of Mr. Labrador criticizing Mr. Trump during the 2016 primary, when he was backing Senator Ted Cruz, and steered it to the West Wing.
The endorsement was off. The day after Idaho’s primary, Mr. Trump phoned the triumphant Mr. Little and, unaware of the tape’s genesis, asked: “Did you see that video?”
And in some cases, Mr. Trump upended his party’s well-laid plans, only to change his mind or lose interest. In Florida, he endorsed Ron DeSantis, a member of Congress who defended him frequently on Fox News, for governor. Yet when Mr. DeSantis broke with the president, over Mr. Trump’s false claim that the death toll in Puerto Rico after Hurricane Maria had been nefarious inflated, the president turned on his handpicked candidate.
Mr. DeSantis ultimately won the governorship by a tiny margin, but Mr. Trump criticized him bitterly in the final days of the race. The former congressman, he complained to a friend, “was low-energy.” Mr. DeSantis even earned a nickname: “Little Ronny D.”