A state representative and a right-wing pastor advanced to a runoff on Tuesday in the Republican primary to replace Representative Kevin Hern of Oklahoma, according to The Associated Press, setting up a battle that will test the strength of President Trump’s influence on the state’s G.O.P. voters.

The state representative, Mark Tedford, and the pastor, Jackson Lahmeyer, will face off in an Aug. 25 runoff in the state’s First Congressional District, a solidly red, Tulsa-area seat.

Mr. Trump endorsed Mr. Lahmeyer in early May, calling him a “MAGA Warrior” who had “been with me from the very beginning of our Movement.” The candidate helped mobilize evangelical support for the president before the 2024 election. Speaker Mike Johnson, Republican of Louisiana, also put his weight behind Mr. Lahmeyer.

Mr. Lahmeyer has deep ties to Mr. Trump’s longtime ally Roger J. Stone Jr., who backed the pastor in his unsuccessful run to unseat Senator James Lankford of Oklahoma in the 2022 Republican primary.

This year’s race was rocked in its final days by a report in The Daily Mail that Mr. Lahmeyer had sent intimate messages to a former campaign fund-raiser who was not his wife, calling her “cute” and floating an invitation to his hotel room. In a Facebook post on Sunday night, the pastor acknowledged sending the messages but said they had been “carefully cherry-picked to create an impression that is not accurate.”

“I own crossing a boundary line through text messaging,” Mr. Lahmeyer wrote in the post. “I also ended all communication.”

Mr. Tedford ran on his business bona fides and his experience as a State House representative, highlighting his endorsement from Kyle Hilbert, the Republican speaker of the Oklahoma House. Mr. Tedford lent more than $1 million to his campaign, significantly outraising Mr. Lahmeyer, according to campaign finance filings.

The winner of the race between Mr. Lahmeyer and Mr. Tedford will be the favorite to replace Mr. Hern, a Republican who has served in the U.S. House since 2018. The congressman gave up his seat to run for the Senate after Mr. Trump selected Markwayne Mullin, then a senator, to lead the Department of Homeland Security.

The Republican nominee will face John Croisant, a Tulsa school board member and businessman, in November. Mr. Croisant, who ran unopposed for the Democratic nomination, has centered his campaign on health care affordability, education and stopping “corruption and chaos,” he said in a recent interview with The New York Times.