MENDON, Ill. — Standing beside former President Donald J. Trump at a packed rally on a sweltering evening here in the fields of west-central Illinois, Representative Mary Miller roared out the stakes of her primary election.
“My friends, this race is between MAGA and a RINO establishment member,” Ms. Miller said, using the unapologetic acronym associated with Mr. Trump’s political movement and the disparaging one meant to tarnish a “Republican in name only.”
Four years ago, it was Ms. Miller’s primary opponent, Representative Rodney Davis, standing alongside Mr. Trump to receive his endorsement when the then-president came to the state to rally his supporters.
But that was before the Democratic-led Illinois legislature gerrymandered the state’s congressional districts, transforming Mr. Davis’s purple district, once a top target of Democrats, into a deeply conservative one that spans roughly a third of the state, and leaving Ms. Miller without a seat.
Now, the two Republicans find themselves pitted against each other in an extraordinary incumbent-versus-incumbent battle that has forced Mr. Davis to embrace his conservative credentials — after nearly a decade when they were a political liability in a district evenly divided by Republican and Democrats — and left him open to attacks from Ms. Miller, who has ridiculed his efforts to reach across the aisle to pass legislation and willingness to certify President Biden’s 2020 electoral victory.
The contest, which comes to a head in Illinois’s Tuesday primary, is a test of which is the stronger force in today’s Republican Party: Mr. Davis’s traditional conservatism and pragmatic style, or Ms. Miller’s inflammatory appeal, with Mr. Trump as her patron, to the hard-right flank.
“Do they want somebody who is going to stick to his or her core values and principles, but also go out and govern?” Mr. Davis asked in an interview at his Springfield campaign office. “Because there’s a distinct difference between my opponent and me when it comes to a record of actually legislating. I want Washington to actually work for every single American.”
Ms. Miller and Mr. Davis’s careers in Congress are a study in contrasts. Mr. Davis, a four-term congressman who got his start in politics working in constituent services, champions his legislative record and his mastery of the farm bill, a multiyear law that allows policymakers to set priorities for the food and agriculture sectors and a crucial piece of legislation in a mostly rural district.
He is deeply conservative, punctuating his remarks with asides condemning the defund the police movement and Speaker Nancy Pelosi, Democrat of California. But he has managed to fend off Democratic challenges for years by touting his bipartisan work on issues including agriculture and student loans.
“The difficulty in that race is Rodney ran in a 50-50 district for the last eight years,” said Representative Darin LaHood, Republican of Illinois, who has endorsed Mr. Davis. “He had to be a moderate. He had to govern in the middle. And so to pivot and then go to one of the most conservative, rural Trump districts in the country is really tough for him.”
For Ms. Miller, whose campaign did not respond to requests for an interview or comment, no such pivot is even necessary.
A first-term congresswoman who owns a cattle and grain farm, she is a member of the far-right Freedom Caucus who has adopted Mr. Trump’s grievance-infused manner of speaking and once spoke approvingly of Adolf Hitler. On the campaign trail, she has made the former president’s endorsement the centerpiece of her pitch and frequently rails about how the “frauds” in elected office have “betrayed” the American people.
At the rally here on Saturday night with Mr. Trump, Ms. Miller’s campaign played videos of Mr. Davis wearing a mask at the height of the pandemic, saying he was “proud” to meet with Mr. Biden to discuss infrastructure projects to benefit his district, and embracing Representative Liz Cheney, Republican of Wyoming, who has helped to lead the House investigation of the Jan. 6 attack on the Capitol.
“The global elites are determined to destroy our way of life, including the family farm,” Ms. Miller told the crowd. “We will not let them destroy us. We are Americans. This is our beautiful country, and we will never surrender to the Marxists in Washington.”
Later in the speech, Ms. Miller called the Supreme Court’s decision on Friday to strike down Roe v. Wade a “victory for white life” in a clip that circulated widely after the rally. Ms. Miller’s campaign said she misread her prepared remarks and meant to say “right to life.” But it recalled an earlier episode shortly after Ms. Miller was sworn in to Congress, when she was forced to apologize for saying: “Hitler was right on one thing: He said, ‘Whoever has the youth, has the future.’”
On Monday, she sought to defend herself from a flurry of criticism after her remarks at the rally, telling a local radio station: “I’m not a racist.”
To rally the kind of hard-right voters who turn out in primary elections, Ms. Miller has also claimed that Mr. Davis “betrayed” Mr. Trump on Jan. 6, first by refusing to overturn Mr. Biden’s electoral victory, and later by voting with 34 of his Republican colleagues to establish a bipartisan Jan. 6 commission made up of nonpartisan experts to investigate the attack on the Capitol.
“He voted to certify the election,” Ms. Miller told a crowd of retirees at a campaign event in Lincoln, outlining Mr. Davis’s perceived sins. “Then, for those of us who were calling for audits, he said we were spreading misinformation.”
Mr. Davis, in his role as the top Republican on the House Administration Committee, had initially worked with Democrats to set up an independent commission to investigate the Jan. 6 riot, but Republican leaders ultimately walked away from that effort and opposed the creation of such an inquiry, prompting Democrats to form their own select panel.
As his primary contest has heated up, Mr. Davis has become increasingly vocal in criticizing the select committee, accusing its members of pushing a “one-sided debate” and inaccurate allegations about Republican lawmakers taking their constituents on tours of the Capitol ahead of Jan. 6 to study the layout of the building.
He said that accusation “literally makes my blood boil.”
But Ms. Miller has ignored such nuances on the campaign trail, telling voters that Mr. Davis “voted for the witch hunt Jan. 6 commission.”
“He doesn’t have any good endorsements,” she added.
In fact, Mr. Davis has been endorsed by 31 of the 35 Republican county chairmen in the district, two out of three of the Republican members of the state’s congressional delegation and the state’s Farm Bureau, all nods that in most races would be seen as critical. But Ms. Miller was likely hinting at one endorsement in particular: Mr. Trump’s.
“I’ve seen Congresswoman Miller in action a lot during this campaign at several different events,” said Tim Butler, a state senator who is supporting Mr. Davis. “The only thing she talks about is Trump. That’s the only thing she talks about. And that’s great. President Trump continues to have wide popularity within Republican circles. But if that’s all you’ve got — I think that’s indicative of how shallow the campaign is.”
Still, it may be enough for many Republican primary voters, especially in the newly drawn, deeply conservative district. Several attendees at Saturday night’s rally said they planned to vote for Ms. Miller, but didn’t know enough about her to feel comfortable giving an interview about why they supported her.
“She’s Trump-endorsed — that’s good enough for me,” said one man who declined to give his name, who was wearing a shirt adorned with a picture of Mr. Trump’s face and the caption “Miss me yet?”