Tim Michels, with Trump’s backing, is Wisconsin Republicans’ choice for governor.

KAUKAUNA, Wis. — Tim Michels, a millionaire construction magnate endorsed by former President Donald J. Trump, won Wisconsin’s Republican primary for governor, according to The Associated Press.

He beat former Lt. Gov. Rebecca Kleefisch and Tim Ramthun, a state assemblyman, in a campaign often dominated by voting issues and the 2020 election.

Mr. Michels entertained the prospect of trying to overturn that year’s result once he is in office, and Mr. Ramthun centered his bid on a proposal to decertify the state’s 2020 presidential election results — a legal impossibility that nonetheless has become a popular cause among Wisconsin Republicans.

Mr. Michels will now face Gov. Tony Evers, a Democrat, in a general election that will determine the fate of voting access in Wisconsin. Mr. Michels has pledged to restrict absentee voting and eliminate the bipartisan Wisconsin Elections Commission; Mr. Evers has vetoed more than a dozen Republican-passed bills to change the state’s voting laws.

A Wisconsin native who inherited an infrastructure company from his father, Mr. Michels lost races for the State Senate in 1998 and the U.S. Senate in 2004 before relocating his family to Greenwich, Conn. He returned to the state this year as he contemplated running for governor.

In the final weeks of the race, Mr. Michels, who largely self-funded his campaign, struggled with questions surrounding his loyalty to Mr. Trump.

During a televised debate in July, he said he would not make decertifying the 2020 election results a priority in his administration — a position at odds with Mr. Trump’s repeated wishes that President Biden’s victory in the state be overturned. Mr. Michels soon began saying that he would consider any legislation the Republican-controlled Wisconsin Legislature sends to his desk.

Then, last week, Mr. Michels said during another debate that he wouldn’t endorse Mr. Trump in a 2024 presidential campaign. Less than 24 hours later, he reversed himself, telling a crowd in Kaukauna that he would back the former president on the day Mr. Trump announced his candidacy.

Still, Mr. Michels triumphed in large part thanks to his endorsement from Mr. Trump — a fact he repeated in television advertising that cost millions of dollars. Ms. Kleefisch was the choice of the state’s Republican political establishment, with endorsements from dozens of state legislators, members of Congress and former Gov. Scott Walker, with whom she served for eight years before losing to Mr. Evers in 2018.

Ms. Kleefisch began a shadow campaign for governor nearly the moment after she and Mr. Walker left office. Though broadly popular while in office, she did not consolidate party support and drew two high-profile and well-funded primary opponents: Mr. Michels and Kevin Nicholson, a businessman whose campaign was funded by the far-right billionaire Richard Uihlein.

Mr. Nicholson dropped out of the race in early July, citing low polling numbers.