Whatever the result of this election, Democrats will be in a moment of transformation afterward. For nearly a decade, the party’s energies have been devoted to defeating Donald Trump. That common goal caused most Democrats to rally around Kamala Harris as soon as Joe Biden stepped aside. But their united front has masked real fissures in the party, and at the intersection of many of them sits John Fetterman. Fetterman, Pennsylvania’s junior Democratic senator, gained early political fame as the towering and tattooed mayor of a working-class town, Braddock, Pa., and then became lieutenant governor of the state in 2019....