Sean Kilkenny wasn’t expecting to be involved in Pennsylvania’s U.S. Senate race last year. The Montgomery County sheriff had backed one of John Fetterman’s rivals in the primary and — like many elected Democrats — didn’t have much of a relationship with the state’s larger-than-life lieutenant governor, who fashioned himself an anti-politician. But as Fetterman’s lead in the polls over Republican Mehmet Oz slipped amid an onslaught of ads attacking him as being soft on crime, the Democrat’s aides reached out to Kilkenny with an urgent message: The campaign needed someone with a law enforcement...