WASHINGTON — President Trump-backed Rep. Randy Feenstra conceded to businessman Zach Lahn in the Iowa GOP primary for governor, marking a rare endorsement defeat for the president in the primaries. Lahn barely edged out Feenstra (R-Iowa) in the contest to take on Democratic state auditor Rob Sand in the general election.

Zach Lahn, a businessman and farmer, narrowly defeated Representative Randy Feenstra in the Republican primary, delivering one of the most significant setbacks to Trump’s political operation since his...