The inside of the beige stucco house in this California desert town seems like a casual place. There’s a gray sectional couch in the den. The fridge is stocked with La Croix. The TV is tuned to Fox News.

But if you look around a little, you’ll see that this isn’t an ordinary house. There’s the bank of phones set up on tables in the dining room. The garage is filled with campaign signs. Then there’s the enlarged Trump tweet posted above the toilet: “Paul Cook is a decorated Marine Corps Veteran who loves and supports our Military and Vets. He is Strong on Crime, the Border, and supported Tax Cuts for the people of California. Paul has my total and complete Endorsement!”

This isn’t just a house. It’s the nerve center of the reelection campaign of Paul Cook, the Republican congressman who represents California’s 8th Congressional District, one of the largest, geographically, in the state. Seated on the sectional couch wearing shorts, a T-shirt, and a ball cap, Cook’s campaign manager, Matt Knox, acknowledges that it’s unusual to run a congressional reelection campaign out of a buddy’s house. But this is an unusual district and an unusual race.

It is the only congressional race in the country pitting two Republicans against each other. California’s “jungle primary” system advances the top two vote-getters, regardless of party, to the general election. Usually, that means voters choose between a Democrat and Republican or, since the state leans so far to the left, two Democrats. Only rarely do two Republicans win the top spots.

In this sprawling conservative district, which runs from the Mojave Desert to Death Valley to just east of Yosemite, that dynamic is producing an odd campaign. President Trump plays prominently in many of this fall’s congressional races, but not usually like this: Cook, the incumbent, is flaunting Trump’s endorsement. But his challenger, Tim Donnelly, is running as Trump’s doppelgänger.

Donnelly, 52, is a former California assemblyman who was an early leader of the Minutemen, a group of fatigues-wearing volunteers who help patrol the southern border. He called for building a wall years before Trump, at a time when Trump was still filming early seasons of The Apprentice and donating to Hillary Clinton’s Senate campaigns.

Donnelly is an outspoken and attention-getting figure, quick with a sound bite and prolific on social media. He has a photo of himself with Diamond and Silk pinned atop his Twitter page, and his Facebook page has a photo of himself with Don Trump Jr. Ann Coulter is coming to town for a fundraising dinner and book signing later this month. He’s been endorsed by Senator Rand Paul and by Turning Point USA founder Charlie Kirk.

Much less flashy is Cook, 75, the incumbent running for a fourth term. He’s a retired Marine colonel who served in Vietnam and is more a standard-issue Republican. Like many of his colleagues, Cook endorsed Trump only after Trump had locked up the nomination, and he skipped the 2016 Republican convention. He tends to side with party leadership, which means he usually votes for spending bills that are unpopular with some conservatives. He’s been endorsed by the National Rifle Association, pro-life groups, and many local elected officials.

The choice poses a baffling dilemma for conservatives following the race: Do they back the challenger who’s like Trump or the incumbent who has backed Trump?

“It kills me,” says Aaron F. Park, 47, an insurance salesman who runs the blog RightOnDaily, which follows state and local politics. “I’m a hard-core conservative, and my heart is with Donnelly. But you don’t know what you’re getting with him. He’s so volatile. . . . Donnelly doesn’t have a filter. Sometimes he’ll say outrageous things to get an audience.” He’s backing Cook because, he says, there are too many red flags in Donnelly’s background. Cook is more moderate, but at least he has backed Trump’s “Make America Great Again agenda,” Park says.

Park’s blog links to an anti-Donnelly site that calls the candidate a “jobless office-seeker” and “convicted criminal.” After serving two terms in the assembly, Donnelly ran unsuccessfully for Congress and for governor. He also pleaded guilty in 2012 to carrying a loaded .45-caliber Colt handgun through airport security. He said at the time that he had placed it in his laptop bag to keep it safely away from his family and forgot to remove it before heading to the airport. He said he tended to be armed to protect himself, since he had received death threats for being a prominent opponent of a California bill to give in-state tuition to illegal immigrants.

More recently, CNN has been on Donnelly’s case over controversial social-media posts and comments he made during a stint as a talk-radio host. In May, CNN reported that Donnelly tweeted that David Hogg, the outspoken student who is pushing gun control after his Parkland, Florida, high school was the target of a mass shooting, had a “Hitlerian fetish to disarm Americans” and was a “#FakeParklandSurvivor.”

Last month, CNN reported that on his radio show in 2015, Donnelly referred to “Ayatollah Obama”: “I don’t know what his connection to the Muslim Brotherhood is, but he’s got them in all kinds of positions of power.” After the 2015 San Bernardino shooting, Donnelly said the Obama administration should “put the mosques on notice. You’d tell them, ‘If you don’t turn over the extremists and jihadists within your midst then we’ll set the FBI on you. We’ll turn your mosques upside down. We’ll make your lives a living hell until you rat out those who have declared war on America.’ ”

Donnelly didn’t back down, telling CNN: “You want to hit me for being too harsh on the a—holes who killed my neighbor and the worthless politicians like Paul Cook, who colluded with Obama and the #FakeNewsMedia to let them into our country.”

Real-estate company owner Becky Otwell, 75, says she favors Donnelly because he seems more committed to Trump’s agenda. She says his campaign seems more energetic.

“I just think we need somebody who is going to get behind Trump and help him,” she says. She wants somebody who doesn’t reluctantly back Trump, but somebody who will “really work,” like Devin Nunes, the chairman of the House Intelligence Committee, which has examined whether intelligence agencies conducted surveillance of the Trump campaign.

She and other Donnelly supporters figure Trump subordinates must have coordinated the president’s endorsement of Cook, since Trump has never gotten to know Donnelly.

In the district, there doesn’t seem to be much of a traditional campaign going on. There are no debates scheduled, no TV ads, and few public appearances. In a large, rural district, campaign signs play an outsized role. Cook calls attention to his status as a retired colonel, which plays well in a district home to Fort Irwin National Training Center and the Marine Corps Air Ground Combat Center, also known as 29 Palms. Cook’s background plays well with voters like retired businessman Thurston “Smitty” Smith, 60, who says he values Cook’s integrity and military service. Donnelly’s signs include a drawing of a Revolutionary War minuteman holding a rifle, with the slogan “Patriot, Not Politician.”

The district voted 55 percent for Trump in 2016. This year, voters who don’t like Trump will face their own quandary. Ordinarily, Democrats and independents would be more likely to vote for the more moderate candidate, Cook. That’s one of the reasons California adopted its primary system, to encourage middle-of-the-road candidates. Yet Trump’s endorsement of Cook could steer some anti-Trump voters to Donnelly. Or maybe large numbers simply won’t vote in the race.

Democratic groups are refusing to endorse either candidate. James Albert, president of the San Bernardino County Young Democrats, says his organization is focusing on electing progressives in other races. “The top-two primary has left people feeling underrepresented in a lot of our communities,” he says. “I would encourage Democrats and independents to focus their attention and energy on down-ballot races.”

Cook would seem to have the edge in both organization and money. He actually has faced another Republican in a general election in the past, in 2012 when he was first elected. He won by 14 points. He beat Democratic challengers by 36 points in 2014 and by 24 points in 2016.

Jack Pitney, a political science professor at Claremont McKenna College, says the race is Cook’s to lose: “So long as he avoids speaking in tongues or showing other signs of possession, he’s fine.” Others aren’t so sure, likening the race to a party primary in which low turnout could make results unpredictable.

There appear to be few animating local issues. Federal land-use policies have traditionally been important, since the district includes Joshua Tree National Park, Mojave National Preserve, and Death Valley National Park. Off-road racing on federal land is a popular pastime.

“The biggest thing you hear from voters is, ‘Does he support the president?’ ” campaign manager Matt Knox says. At 10 on a recent weekday morning, a half-dozen young volunteers walk in the front door of the campaign headquarters house in Victorville. They walk through the kitchen to the phones and start dialing to let people know that for Cook, the answer is an unqualified “yes."