Rep. Thomas Massie (R-KY) is facing the toughest primary challenge of his political career as President Donald Trump wages an aggressive campaign to oust one of his most outspoken critics from Congress.Massie will face Trump-backed Ed Gallrein on May 19 in a closely watched race that could test the limits of Trump’s grip on Republican voters. “It’s going to be a close race,” Massie told the Washington Examiner, arguing the contest would hinge on turnout. According to Massie, younger Republicans have gravitated toward him while older GOP voters have lined up behind Gallrein.

Massie has spent more than a decade cultivating a reputation as one of Congress’s most independent Republicans. A Tea Party libertarian, he frequently bucks both parties on spending and foreign policy and has emerged as one of the few GOP lawmakers willing to directly oppose Trump during his second term.

“I think it’s hard to get motivated to go vote for nobody against somebody, but I think it’s easier to be motivated to go vote for somebody you believe in. Like nobody really believes in my opponent,” Massie explained.

Massie has survived primary challenges before, but this race is his most difficult. Outside groups aligned with Trump have blanketed Kentucky airwaves with more than $25 million in ads boosting Gallrein and blasting Massie.