A demonstrator in a Donald Trump mask stands outside a campaign event held by Republican Senate candidate J.D. Vance, and the former president in Independence, Ohio.MARK PETERSON/The New York Times News Service Edward McClelland is the author of Midnight in Vehicle City: General Motors, Flint, and the Strike That Built the Middle Class, and an editor at Chicago magazine. The night before he won the Republican nomination for governor of Illinois, State Senator Darren Bailey held a rally at the Des Plaines Theatre, a converted opera house just outside Chicago. A hundred or so blue-jeaned Baby Boomers in “Mama Bears...