Facing a tough election challenge is a cost of doing business for members of Congress. But when that challenge comes from a colleague, things can get personal quickly.

That’s what’s happening right now in Southern California, where tensions between Republican Reps. Ken Calvert and Young Kim are boiling over after Democratic redistricting forced them into a fight for their political lives ahead of June’s primary.

Kim is panning her colleague, a 30-year veteran of Congress, as a creature of Washington. Calvert is calling her insufficiently conservative on issues like immigration. And Calvert even accused Kim, after the recent attempt to assassinate President Donald Trump, of contributing to “the heated environment and violence we’ve seen” because she has criticized Trump in the past, leading Kim to retort that Calvert is running a “desperate” campaign.

The messy intraparty fight is a result of state Democrats’ aggressive redistricting efforts last year, which carved up Republican-held seats in Southern California to create more blue-leaning districts. Along the way, they crammed into one red district a powerful member of the House Appropriations Committee, Calvert, and a battle-tested incumbent who has won tough races in recent years, Kim.

“Love and war have rules,” said Rep. Darrell Issa, a retiring Republican also from Southern California. “Politics doesn’t.”

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Kim, a former congressional aide and state lawmaker, first ran for Congress and lost in the 2018 blue wave. But she won the seat two years later even as Trump lost re-election and Joe Biden carried the district by 10 points.

Her past campaigns were defined in part by the distance she sought to keep from Trump. She told the Los Angeles Times in 2018 she was “not running for the party of Trump” and criticized the administration’s family separation policies. During the pandemic, she panned Trump’s use of the term “Kung Flu” as “a very insensitive remark,” and she sponsored a resolution to censure Trump after the Jan. 6, 2021, Capitol riot, though she voted against impeachment.

But that’s all changed.

Kim, a prolific fundraiser, is now on the airwaves with ads declaring, “Young Kim backs President Trump 100%,” and calling herself the “trusted Trump conservative.”

“I may be in my third term only, but [it’s] my first term in office with President Trump in the White House, and I’m excited that I’ve been able to do so much in the short period of time working with President Trump to secure our border, support our law enforcement and getting our economy back on track she said,” she said in an interview, noting that she traveled to Trump’s Mar-a-Lago resort in Florida with some colleagues to meet with him about legislative priorities, like increasing the cap for state and local tax deductions, before he took office again.

“I am backing President Trump 100%,” she added.

The Calvert campaign and its allies have hammered Kim for her rhetorical shift in campaign ads of their own.

“She’s trying to reinvent herself in the sense of — 100% with President Trump? That’s not true,” Calvert said in an interview. “She tried to censure the president. She said he needed to be punished. ... She spent 10 years running away from him.”

Statements from Calvert’s campaign have gone further. Days after the assassination attempt at the White House Correspondents’ Association dinner, Calvert’s campaign sent out a release panning “Kim’s dangerous anti-Trump rhetoric,” connecting her past criticism to “the out-of-control political rhetoric that has poisoned our politics.”

“Trump-hating politicians like Young Kim think their inflammatory language doesn’t matter but it’s led to the heated environment and violence we’ve seen,” Calvert said in a statement alongside last week’s news release.

Kim told NBC News she hadn’t seen that specific criticism from Calvert. After she heard part of the campaign statement, she called the accusation “dangerously desperate” and “absurd,” pointing out that she has introduced a resolution to condemn the attack and political violence generally. She also noted that prominent Republican allies of Trump’s have criticized his rhetoric in the past, too.

“Many of the people serving in his current administration have also disagreed with the president’s rhetoric. That’s not a secret here. This is just another desperate attempt by Ken Calvert because he cannot run on his 30-plus-year record in office,” she said.

The campaigns are also in an ad war — with Kim’s claiming Calvert falsely implied Trump had endorsed him. Calvert told NBC News that his ad emphasizes that he has previously received Trump’s blessing.

Kim’s pivot to Calvert’s decades in Washington typifies the ground she wants to fight the race on.

“This race is ultimately going to come down to how long someone needs to be in office to fix things,” she said. “Is 30 years in office enough time to get things done? If not, how many more decades does Congressman Calvert need to get things done?”

Calvert said he’s happy to fight on those terms, especially since his long career in Congress means he has represented much of the district in the past. A narrow majority of residents in the new 40th District are from Calvert’s current district, according to data compiled by The Downballot, while about a third come from Kim’s current district.

Calvert was one of the initial proponents of the E-Verify system that checks employee citizenship status, and he is leaning on not just his ardent support for Trump over the years but also his conservative chops on issues like immigration.

He has panned Kim for supporting a bipartisan immigration compromise bill that combines a pathway to lawful permanent status for certain immigrants currently in the U.S. illegally with other policies like increased penalties for repeated attempts to enter the country. He also repeatedly emphasized his role as the head of the House committee tasked with outlining the defense budget, stressing the importance of seniority in delivering for his constituents in an area flush with defense contractors.

“Nobody has delivered more resources back to the state of California than I have, in either party,” Calvert said. “It takes a long time to get to the position I’m in and the responsibility that I have, and it has a lot of weight here in California. We have more military installations than any state in the union, about 41 of them.”

For all the intraparty fighting, California’s unique primary system — which pits every candidate on the same ballot regardless of party, with the top two moving on to a general election — could mean the Republican-on-Republican battle may not be over on primary day. Whether Calvert and Kim or a Republican and a Democrat move on to the fall general election could throw yet another wrinkle into the race.

Dave Gilliard, a longtime California Republican strategist who isn’t working in the race, told NBC News that while Kim’s biggest vulnerability now is her history with Trump, “you may see her shift back toward the middle with a whole new group of voters to compete with” if the two Republicans both advance to the general election after the top-two primary — and that she could benefit from appealing to a broader general electorate.

“Calvert’s best play is to get into a runoff with a Democrat, and then this race is over on June 2,” Gilliard added.

As the tensions between the two House colleagues remain high, Calvert noted that he donated to Kim’s campaign last year, before redistricting made them foes.

“I maxed out to her election, gave her $10,000,” he said. “I asked for the money back, but she wouldn’t give it to me.”