Representative Young Kim, a battle-tested Orange County Republican, has ousted veteran Democrats in tight races, but she’s actually a “Trump-hating liberal,” one television advertisement proclaims.

Representative Ken Calvert, a Republican fixture in nearby Riverside County, is a longtime thorn in the side of Democrats, but he’s guilty of “sabotaging President Trump’s agenda,” a different ad declares.

Some Republicans are distancing themselves from Mr. Trump these days, given his flagging approval ratings and an unpopular war with Iran that has divided the MAGA base. But not Ms. Kim or Mr. Calvert, two G.O.P. incumbents forced to face off for political survival after Democrats scrambled their House districts in a gerrymandering push last fall.

Squaring off in a redrawn and heavily Republican district that combines their turf, Ms. Kim, 63, and Mr. Calvert, 72, longtime colleagues in the House, are both proudly touting their conservative bona fides while accusing each other of being out of step with MAGA voters.

For all the hand-wringing over fractures in Mr. Trump’s America First coalition and his waning influence over some corners of the Republican Party, the contest shows that plenty of conservatives remain eager to align closely with the president.

Both members say they maintain a cordial relationship when they see each other on Capitol Hill. But as a June primary approaches, there is little to call cordial in their campaign rhetoric. Mr. Calvert, in particular, has blasted out a series of emails portraying Ms. Kim as a RINO: a Republican In Name Only.

“This race is going to get very ugly, very quickly,” said Jon Fleischman, a veteran Republican strategist in Orange County who has not taken a side in the primary. He added: “These are both rough-and-tumble candidates.”

This primary season has seen a host of intraparty spats, but most have featured Democratic incumbents facing challengers as the party endures a reckoning over age and establishment politics. Last year’s redistricting wars, though, have set up several potential member-on-member contests, including two Democratic representatives in Houston.

Republicans have ducked potential face-offs created by redistricting elsewhere, in Utah and in Northern California.

But in Southern California, Ms. Kim and Mr. Calvert are clinging doggedly to shrinking red turf. They are fighting over the new 40th District, a swath of terrain southeast of Los Angeles that includes Orange County suburbs, Riverside County cities and a stretch of the Santa Ana Mountains near where bright orange California poppies sometimes form a vibrant spring “super bloom.”

Proposition 50, the redistricting measure championed by California Democrats as a response to Republican-led redistricting in Texas, imperiled five Republican-held districts — including Mr. Calvert’s, which was sliced up and divided among other nearby districts. In the process of making other districts more favorable to Democrats, the ballot measure dumped conservative neighborhoods into the 40th District held by Ms. Kim, transforming it to a solidly Republican seat from a battleground. Because it includes significant territory from Mr. Calvert’s current district, both incumbents can lay claim to it.

The G.O.P. infighting has divided local Republicans, who must confront the question: Who do you side with when two of your friends are fighting?

“It’s very awkward — it’s awkward for donors, it’s awkward for electeds,” said Will O’Neill, the chairman of the Republican Party of Orange County. “Because our county party hasn’t endorsed between two good Republicans, my job is to make sure the conflict doesn’t cause down-ballot problems.”

Mr. Calvert, a 33-year House veteran who leads the powerful defense appropriations subcommittee, has cultivated decades of local connections. Ms. Kim, known as a prodigious fund-raiser and tenacious campaigner, has become battle-scarred with her narrow loss in a battleground House district to Gil Cisneros, a Democrat, in 2018, and then her comeback victory against Mr. Cisneros two years later.

In this year’s showdown, a super PAC aligned with Mr. Calvert struck first, dropping about $2.2 million into television, radio and mail advertisements this spring accusing Ms. Kim of being insufficiently conservative.

The super PAC does have some fodder to work with: Ms. Kim voted in 2021 to censure Mr. Trump for his role in inciting the Jan. 6 Capitol riot. Mr. Calvert’s campaign has also pointed to Ms. Kim’s comments to the The Orange County Register in 2022, when, asked about Mr. Trump, she said she had “never met him.” (Ms. Kim’s campaign said the two have met on several occasions since then, pointing to evidence of two encounters in early 2025.)

“We need a conservative who’s been with President Trump through thick and thin, not just when it’s politically expedient,” Mr. Calvert said in an interview. “She’s part of the more moderate group in the House, and she has separated herself from President Trump on numerous occasions.”

He added: “I don’t try to make this stuff personal, but sometimes it ends up that way.”

Ms. Kim, who announced a $3.7 million advertising push last month, argued that she had always been very conservative, and said her votes showed a record of accomplishment and support for Mr. Trump.

It was Mr. Calvert, she suggested, whom voters needed to look at with skepticism.

“After 30-plus years in Washington, are things actually better?” Ms. Kim asked in an interview. “What good is seniority if it hasn’t delivered results and accountability?”

Because Mr. Calvert had spent so long in Washington, she argued, he represented the antithesis of the anti-establishment sentiment that fueled Mr. Trump’s return to the White House.

“Being in office for this period of time is actually going against the MAGA movement,” Ms. Kim said. “Time’s up!”

Still, she added: “We’re very cordial and we respect one another.”

Mr. Trump’s presence continues to hang over the race, even though he has not endorsed in it.

Mr. Calvert’s campaign recently pointed to a video from an event in 2020, where Ms. Kim told attendees: “As a mom, if you asked me, I would not let any one of my three daughters date or marry somebody like Donald Trump.” She added: “I’ll whip his butt, if I can.” (She did also praise his leadership as “refreshing” later in the video clip.)

Ms. Kim’s campaign, meanwhile, highlighted the fact that Mr. Calvert’s campaign replaced a recent advertisement implying that Mr. Trump had endorsed Mr. Calvert in his current re-election bid. A newer version of the ad does not include the lines suggesting an endorsement.

Because California holds nonpartisan primaries in which the top two candidates advance to the general election regardless of party, both Republicans could make it to the November ballot.

But the X factor may be a Democratic spoiler candidate, who would be a heavy underdog in November but could claim one of the two spots in the June primary if the incumbents split the Republican vote, locking one of them out of the general election.

A handful of Democrats are running in the primary, lessening that likelihood. But Esther Kim Varet, a wealthy art dealer who has out-raised other Democrats in the field, argued that she herself was capable of pulling off an unlikely upset.

“We have an opportunity to eliminate two congresspeople in a very, very thin-margined Congress,” Ms. Kim Varet said. “Right now, they’re having to out-MAGA each other.”

Ms. Kim Varet said that Ms. Kim, who is about 20 years her senior and also Korean American, reminded her of her parents’ generation. Given the dissatisfaction with Mr. Trump’s tenure, Ms. Kim Varet said, there was a desire for generational change among Southern California’s Korean American community.

Ms. Kim, who was born in South Korea and moved to the United States with her parents as a child, said the attacks from a fellow Korean American were “disappointing.”

“She should know how challenging it is for immigrants like us and our families to come to America, to take advantage of every opportunity,” Ms. Kim said. “I wish her well — she can run a campaign the way she’s doing, but I will never attack her for being who she is.”