Donald Trump is set to back Wyoming attorney Harriet Hageman as she prepares a primary challenge against GOP Rep. Liz Cheney, according to three people with knowledge of his plans, marking the most important political endorsement yet in Trump’s post-presidency.

Trump’s looming involvement in the primary will test his political power in the GOP like never before, as he seeks to punish the most high-profile House Republican to vote for his impeachment in January. His allies and team not only encouraged Hageman to run against Cheney — they now are under pressure to clear the crowded primary field of other candidates who could split anti-Cheney sentiment, which would give the incumbent the chance to win her primary with only a plurality…

Hageman isn’t just banking on Trump’s endorsement in the coming primary against Cheney: Top Trump staffers and allies are in her corner, including some who are in talks to occupy key roles on her campaign or with a super PAC prepared to back her. Some former Trump campaign hands and advisers met with Hageman in March at the urging of local conservatives.