Mayor Shawn Reilly of Waukesha, Wis., an independent who was a Republican for most of his life, said in an interview on Wednesday that he was endorsing Vice President Kamala Harris for president.

The endorsement is a key one for Ms. Harris, whose campaign has lavished attention on the suburbs of Milwaukee, which lean Republican but are so densely populated that they deliver a pivotal number of Democratic votes in the swing state.

Mr. Reilly, 63, said that he had never endorsed a Democrat before. But this election is different, he said, describing his own evolution from loyal Republican for decades to an independent in 2021.

“It’s very easy to not even stick your nose in this — that’s the easiest way to go about it,” he said. “But the reason I’m doing it is because I think we’re at a crossroads. I’m very afraid of the direction our country will head in if Donald Trump becomes president. I think we’ll be heading down a road of authoritarianism and fascism.”

The Harris campaign has poured considerable energy and resources into Waukesha County, which includes the city of Waukesha, hoping that Ms. Harris will be able to cut into Mr. Trump’s margins there. In 2020, Mr. Trump won the county with just under 60 percent of the vote. More than 400,000 people live in Waukesha County, the third-most populous county in Wisconsin, behind Milwaukee and Dane.

Since 2014, Mr. Reilly has been the mayor, a nonpartisan role. When asked during his initial mayoral campaign if he was a Republican, he always answered yes, he said.

When Mr. Trump ran for president in 2016, Mr. Reilly did not vote for him, but did not vote for Hillary Clinton, the Democrat, either. After the attack on the Capitol in 2021, Mr. Reilly disavowed the Republican Party, saying he no longer considered himself a member.

He attended a rally in Waukesha County on Monday, where Ms. Harris appeared with former Representative Liz Cheney of Wyoming, in an appeal to conservative women in the Milwaukee suburbs.

On Tuesday, early voting began in Wisconsin. From Mr. Reilly’s perch in City Hall in Waukesha, he could see a line of voters snaking down the sidewalk. About 800 people were voting in person each day, he said.