Eileen Laubacher put 40,000 miles on her campaign car visiting ranching towns and rural hospitals across eastern Colorado as she tried to persuade Republican voters to dump their congresswoman, the hard-right firebrand Lauren Boebert.

Ms. Laubacher, a Navy veteran and former Republican, was a first-time Democratic candidate hoping to oust a gun-toting icon of the MAGA movement in Colorado’s reddest congressional district. Even some of her own supporters worried it was a losing fight.

But on Friday, The Associated Press declared Ms. Laubacher the winner in the unexpectedly close race.

She called her win a testament to showing up in rural towns and other places where voters felt ignored and ill-served by both major political parties. But she acknowledged she had a lot of work ahead to keep her Republican-leaning district on her side.

“We have a lot to deliver on,” Ms. Laubacher said in an interview from Washington, where she spent Monday in an orientation with other newly elected House members. “Just because we won this election doesn’t mean things have gotten better for people. Groceries haven’t gotten cheaper. The war in Iran is still happening.”

Ms. Laubacher campaigned on cutting costs, ending President Trump’s tariffs and reducing taxes on the middle class. A retired rear admiral, she pitched herself as a service-minded suburban mother of five and assailed Ms. Boebert as a walking scandal who had harmed farmers and veterans in her rural district.

For Democrats, it has long paid to square off with Ms. Boebert, a conservative lightning rod whose opponents invariably raise donations from Democratic partisans across the country. Ms. Laubacher brought in $13.7 million to defeat Ms. Boebert. The incumbent raised $1.2 million.

Ms. Laubacher is the third Democrat to flip a Republican-held congressional seat in an increasingly blue Colorado. Republicans held four of the state’s eight House seats before the midterms, but the only Republican left standing is Representative Jeff Hurd, who represents Colorado’s rural Western Slope, the district Ms. Boebert first represented but abandoned when she came within 546 votes of defeat in 2022.

Ms, Boebert’s adopted district stretches from Denver’s fast-growing exurbs to farming and ranching towns along Colorado’s eastern plains, a seemingly impenetrable conservative redoubt.

But Ms. Laubacher spent the past year and a half crisscrossing the district, which was hit by the same economic headwinds that softened Republican support in neighboring Kansas. Over the fall, Democrats and political analysts began to think that Ms. Boebert could be vulnerable because of her close ties to Mr. Trump and his unpopular policies on tariffs and the Iran war.

Ms. Boebert clashed with Mr. Trump over the release of government files on the convicted sex offender Jeffrey Epstein and Mr. Trump’s veto of a clean-water project for her parched district. Mr. Trump ultimately endorsed Ms. Boebert and held a tele-rally for her in October.

Ms. Laubacher courted suburban voters frustrated with high prices and Ms. Boebert’s personal antics. She also made inroads with rural voters angry about the high price of diesel and fertilizer.

In her campaign ads, Ms. Laubacher sat at a kitchen table discussing strained family budgets while a split-screen image showed security-camera footage of the evening in 2023 when Ms. Boebert and a companion were kicked out of a Denver theater for disrupting a performance of the musical “Beetlejuice.”

“She is not a serious person,” Ms. Laubacher said of Ms. Boebert.

At times, she sounded more like a moderate Republican than a party-line Democrat. During a debate, she said she would not vote to impeach the president. She said she supported a balanced budget, and called the $40 trillion national debt “terrible and tragic.” On Monday, she declined to say whom she might support for House speaker.

Meg Furlow, the Democratic chairwoman in Douglas County, said Ms. Laubacher showed up everywhere in the district, even when it still seemed like her campaign was a long shot.

On Monday, Ms. Furlow said Democrats in Douglas County, where Ms. Laubacher lives, were all still in shock.

“We’ve always thought it was possible, but maybe a pipe dream,” Ms. Furlow said.

Ms. Laubacher’s campaign said she had visited all 21 counties in the district, knocked on 25,000 doors and held 120 public events. She sat down with voters at green-chile harvests, veterans’ meetings and small Republican towns where Democrats rarely campaign.

Steve Nein, a farmer, retired banker and one of the few Democrats in rural Sedgwick County, near the Colorado-Nebraska state line, said he had been impressed when Ms. Laubacher showed up to speak to a room filled with Republicans and independent voters.

“At least they were listening,” Mr. Nein said. “It was an uphill battle.”

Ms. Laubacher said her campaign had framed a local newspaper article about how many people had attended that meeting. It was one moment when she began to think she might win.“

They were frustrated and felt ignored by elected officials,” Ms. Laubacher said. “They were tired of promises that never materialize. I thought to myself, this is where it’s going to matter.”