Ben Ingebretson was a reliable Republican voter. A Christian minister who works for a faith-based nonprofit, Mr. Ingebretson said he shared Republicans’ small-government views and agreed with the party’s calls for fiscal conservatism. But after Donald J. Trump became the face of the party, Mr. Ingebretson, who lives in Grand Rapids, Mich., drifted away from his political roots. In 2020, he voted for President Biden. And in February, he was among the 34 percent of voters in his county who...