That possibility has unnerved Republicans inside and outside the White House. Some worry about straining the president’s already tenuous relations with congressional Republicans at a time when they face several key challenges this fall: raising the debt ceiling, passing a spending bill and tackling their top policy objective of new tax legislation. Others looking ahead to next year’s midterm elections think Trump may even be putting the Senate GOP majority at risk.

Allies of Flake, including Senate Majority Leader Mitch McConnell (R-Ky.), are vexed by Trump’s posture. At a minimum, they think he is needlessly creating a costly primary that will suck resources away from other targets on a map ripe for gains.

“There are 10 Senate Democrats up for reelection in states the President won in 2016, and that’s where his political focus and energy ought to be over the next 14 months, instead of harmful intraparty warfare,” said Brian Walsh, a former communications director for the National Republican Senatorial Committee.