Sinema’s Senate Seat Draws First Major Republican Contender

Mark Lamb, a right-wing sheriff and an ally of former President Donald J. Trump known for his policing of elections and his defiance of a pandemic lockdown, announced Tuesday that he would run for Senate in Arizona next year, a contest that could determine control of the closely divided chamber.

Mr. Lamb, 50, became the first high-profile Republican to compete for the seat, one currently held by Senator Kyrsten Sinema, who left the Democratic Party in December to become an independent. Ms. Sinema has not said whether she will run, but if she does, there is already one Democratic challenger: Representative Ruben Gallego, a progressive Democrat from Phoenix.

In his announcement video, Mr. Lamb said he will “stand up to the woke left” and “secure our border and support our law enforcement.” He also called out his support for gun rights and his anti-abortion stance in the ad.

Mr. Lamb, as the top law enforcement officer in Arizona’s third-most populous county, Pinal, made headlines when he refused to enforce the state’s stay-at-home order in 2020 and then when he expressed sympathy for the rioters who attacked the Capitol on Jan. 6, 2021. He has also sown doubt over the results of the 2020 election and drawn scrutiny for his embrace of private militias and hard-line positions on immigration.

The field appears likely to grow, as Republicans see an opening to retake the seat in the potential matchup between Mr. Gallego and Ms. Sinema, which could split the Democratic and independent voters who have powered victories for the left in the state.

Kari Lake, a Republican who refused to accept her defeat in the governor’s race last year, has also signaled that she could jump into the race.

Ms. Sinema has infuriated Democrats with her departure and opposition to key planks of their agenda in the Senate. Her split with the party came shortly after it gained an outright majority in the Senate during the midterm elections last fall.

Arizona was one of the key battlegrounds in those elections, and in 2020, Joseph R. Biden Jr.’s victory there over Mr. Trump helped him secure the presidency.