WASHINGTON — Representative Doug Collins, one of the faces of President Trump’s impeachment defense, plans to challenge Senator Kelly Loeffler in this fall’s special election for one of Georgia’s Senate seats, people familiar with his intentions said on Monday.
Mr. Collins’s long-expected decision sets the stage for a brutal Republican-on-Republican fight that will put a prominent House conservative known for his defenses of Mr. Trump against a wealthy businesswoman appointed in recent weeks to fill the state’s vacant Senate seat. Democrats also see the contest as a potential opportunity to pick up a seat in November.
Mr. Collins is expected to announce his campaign on Tuesday, according to the people familiar with his plans, who were not authorized to discuss it publicly.
He enters the race with widespread name recognition on the right, having served as the top Republican on the House Judiciary Committee during its impeachment proceedings and now as a member of Mr. Trump’s impeachment defense team.
Gov. Brian Kemp of Georgia, a Republican, passed over Mr. Collins in favor of Ms. Loeffler when he selected a temporary replacement for the retiring Senator Johnny Isakson late last year. He did so despite direct entreaties from Mr. Trump and some of his allies on Mr. Collins’s behalf, and their complaints that Ms. Loeffler was not a reliable enough supporter of the president.
Ms. Loeffler, a prominent political donor and political newcomer, has spent the weeks since trying to burnish a conservative record. She did so again on Monday just hours before news of Mr. Collins’s campaign surfaced. She attacked Senator Mitt Romney of Utah, a Republican to whom she once donated large sums of money, for saying that witnesses should be called in Mr. Trump’s impeachment trial.
“After 2 weeks, it’s clear that Democrats have no case for impeachment,” she wrote on Twitter. “Sadly, my colleague @SenatorRomney wants to appease the left by calling witnesses who will slander the @realDonaldTrump during their 15 minutes of fame. The circus is over. It’s time to move on!”
She starts with a significant financial advantage. The senator has said she is willing to pour $20 million of her own money into the race.
Ms. Loeffler ignored questions from a reporter in the Capitol on Monday night, and her aides did not respond to a request for comment.
Under the existing rules governing special elections in Georgia, there is no party primary in the race, and both Ms. Loeffler and Mr. Collins could be on a single ballot on Election Day in November along with Democratic candidates. If no candidate won more than 50 percent of the vote, the top two vote getters would compete in a January 2021 runoff.
The Georgia House of Representatives is considering legislation that would institute party primaries for the contest, a change that could benefit Mr. Collins. Mr. Kemp has threatened to veto it, however.
What gives Republicans some comfort about what could be a messy intraparty fight is that Democrats don’t have an obvious candidate to field in the race. A handful of Democrats are already running, including former Senator Joseph I. Lieberman’s son, but state and national leaders have made clear they are still recruiting others.
One possibility Democrats in Washington are said to be eyeing: the Rev. Raphael Warnock, the pastor of the storied Ebenezer Baptist Church in Atlanta.
Mr. Collins’s decision is a rebuke of Mr. Kemp, the ostensible party leader in Georgia, and of Senator Mitch McConnell, Republican of Kentucky and the majority leader, who also backs Ms. Loeffler. But it is not much of a political gamble in an era when Mr. Trump’s favor or opposition is what shapes Republican politics.
The president has made little secret of his fondness for Mr. Collins. Now, though, Mr. Trump finds himself between a loyal lawmaker and a sitting senator who has the backing of Mr. McConnell and Georgia Republicans like Mr. Kemp who are eager to elevate a woman in an increasingly competitive state.
Some White House officials indicated Monday that Mr. Trump might stay out of the intramural fight. But he has inserted himself in past primaries after a nudge. One of those races was Mr. Kemp’s own 2018 primary, when the president endorsed him late in the contest.
But with Republicans finding it harder to keep Georgia in their column, some of the president’s advisers are hoping he does not take sides in a race between two candidates who have made clear they will be White House allies.
Mr. Collins’s decision to abandon his House seat will also have implications in that chamber. Two close allies of Mr. Trump, Representatives Jim Jordan of Ohio and John Ratcliffe of Texas, are the leading contenders to replace him as the top Republican on the Judiciary Committee, a coveted post. Both men helped lead Mr. Trump’s impeachment defense in the House.