Kari Lake won the Republican primary for U.S. Senate in Arizona on Tuesday, according to The Associated Press, setting up a high-stakes contest in the fall for the seat of Senator Kyrsten Sinema, who is retiring.

Her victory over Mark Lamb, the Pinal County sheriff, extends her three-year transformation into a fierce pro-Trump firebrand. A former news anchor, she will now face Representative Ruben Gallego, a Phoenix-area former Marine who had no opposition in the Democratic primary.

Ms. Lake celebrated her victory in front of supporters at a Phoenix hotel, telling them she would be former President Donald J. Trump’s “backup” in Washington after defeating Mr. Gallego. She brought out a thick binder, saying it was a tally of Mr. Gallego’s “destructive voting record that is destroying Arizona.”

Though she made overtures to building a broader coalition — calling for “disaffected Democrats” and moderate Republicans to join her — she framed the election in stark terms.

“This is a battle between good and evil,” she said. “This is a battle between the people who want to destroy this country and the people who want to save America.”

Ms. Lake led Mr. Lamb 53.4 percent to 40.6 percent, with 67 percent of the vote reporting.

Ms. Lake and Mr. Gallego have already spent months attacking each other. The race has been essentially set — Mr. Lamb’s spoiler potential was the only question mark — since March, when Ms. Sinema, who left the Democratic Party in 2022 to become an independent, declared that she would not seek re-election.

Running in a border state, Ms. Lake has accused Mr. Gallego of being a far-left radical and favoring loose restrictions on immigration, while Mr. Gallego has blasted Ms. Lake for her shifting stance on abortion rights and for continuing to make baseless claims of election fraud. On Monday, he committed to debating Ms. Lake, while she expressed doubt about the debate host — the state Clean Elections Commission — but told reporters “our teams can discuss a fair place, a fair platform to do that.”

After her victory, Mr. Gallego assailed her again, saying in a statement that he welcomed Arizonans “to join our team and help defeat Kari Lake and her dangerous plan to ban abortion and hurt Arizonans.”

Both candidates face questions about their ability to pull in independent voters and moderates heading into November, though Mr. Gallego has maintained a consistent, if narrow, lead in most surveys of the race and has a sizable fund-raising advantage.

When Ms. Lake ran for governor in 2022 as a political newcomer, she roiled the Republican Party with an abrasive primary campaign. Her divisiveness and fervent embrace of Mr. Trump’s claims of election fraud helped Katie Hobbs, the Democrat, claim victory. After she lost, Ms. Lake filed a series of fruitless lawsuits, asserting without evidence that the election had been rigged against her.

More mainstream Republicans backed away from Ms. Lake as she continued her legal fights, but they returned to her side as she began her Senate campaign — albeit with encouragement for her to tone down the stolen-election rhetoric. She has worked to mend fences with Republicans and keep her focus on issues like border crossings and the economy, even as she continues her effort to overturn the governor’s race. She is also facing a defamation suit from an official in charge of overseeing elections in Maricopa County.

After Ms. Lake won on Tuesday, her bitter primary rival in the 2022 governor’s race, Karrin Taylor Robson, endorsed her in a statement.

Mr. Gallego has sought to shed his longtime progressive label as he, too, courts Arizona’s center. Democrats in the state have exploited Republican divisions to claim most statewide offices in recent years, but political observers suggest that Mr. Gallego, as an unabashed liberal, could have an uphill climb in attracting enough of the state’s moderate Republicans to his side. He has made some progress so far, securing endorsements from some local G.O.P. leaders and members of the business community, and receiving contributions from longtime Republican donors.

Republicans, with a buffet of vulnerable Democratic senators to challenge, do not need the Arizona seat to reclaim the U.S. Senate. Although the National Republican Senatorial Committee has run advertisement in support of her campaign, it is unclear to what degree the party will continue to aid Ms. Lake.

“Arizonans must unite to defeat Ruben Gallego, one of the most radical Democrats in the country,” Senator Steve Daines of Montana, the chair of the N.R.S.C., wrote in a statement congratulating Ms. Lake on Tuesday.