WASHINGTON — Darrell Issa, the former nine-term congressman from California, will try to return to Congress by challenging Representative Duncan Hunter, who was indicted last year, setting up a clash among two Trump allies for one of Republicans’ most endangered seats.

Mr. Issa is expected to announce his challenge at a news conference on Thursday morning in the San Diego County district, according to two sources familiar with his decision. He had been publicly mulling for months whether to make a bid to return to Congress, and started an exploratory committee in August.

“I have received such a tremendous outpouring of encouragement from supporters inside the district, and around the state and across the nation,” Mr. Issa said in August in a statement posted to the committee’s website.

Mr. Issa will likely bill himself to Republicans as a safer alternative than the incumbent. Mr. Hunter was indicted last August. He and his wife, Margaret, are accused of spending more than $250,000 in campaign funds on personal expenses — including a vacation to Italy, fast food runs and plane tickets for a pet rabbit — and then lying to the Federal Election Commission. Mr. Hunter has remained defiant and has called the indictment a politically motivated attack. His campaign did not immediately respond to a request for comment on Mr. Issa’s looming challenge.