Few midterm races had as much potential to test how voters view President Trump’s immigration crackdown as a contest for a competitive House seat in Ohio, where the former No. 2 at U.S. Immigration and Customs Enforcement announced a bid this winter.

Madison D. Sheahan, who stepped down from ICE to run against Representative Marcy Kaptur, a Democrat, had leaned into her work at the agency, calling herself “a Trump conservative” and highlighting her efforts to stop illegal immigration.

But on Tuesday she fell well short in the Republican primary race, placing a distant third in a race won by Derek Merrin, a former Ohio state legislator, according to The Associated Press.

The primary victory of Mr. Merrin, 40, who called himself a “MAGA Republican,” could give his party more hope of finally unseating Ms. Kaptur, 79, the longest-serving woman in Congress, who has represented the Toledo area in the House since she was first elected in 1982. Mr. Merrin lost only narrowly to Ms. Kaptur in 2024, and Ohio Republicans have since redrawn her seat in the Ninth Congressional District to be slightly friendlier to their party.

Mr. Merrin topped a crowded field that included Ms. Sheahan, who served as a deputy director at ICE under Kristi Noem, the former homeland security secretary, as well as Josh Williams, a state representative, who finished second.

Ms. Kaptur has long been resilient — in 2024, she was one of just 13 Democrats who won House districts in which voters backed Mr. Trump — but Ohio’s new congressional map has put her in a more precarious position. While Democrats are favored to pick up House seats in this fall’s midterm elections and potentially take back the chamber, Ms. Kaptur’s seat is among the party’s most endangered.

Mr. Trump did not make an endorsement in the Republican primary race, though Mr. Merrin and the other leading Republican candidates all sold themselves as allies of the president.

Mr. Merrin, who has worked as a real estate agent, has been in politics his entire adult life. He has served as mayor and on the City Council of Waterville, Ohio, then worked in the state auditor’s office and spent eight years in the Ohio Statehouse.