In a cramped banquet room on the top floor of Janesville's historic Armory building, a contender for Wisconsin's state Senate tells an old joke—the one about politicians and diapers needing to be changed often, and for the same reason. "These politicians start to stagnate after awhile," the candidate, a businessman named Brian Fitzgerald, tells the crowd of local Republicans. Just then, a voice booms from the back. "Brian! I'm right here!" The room erupts in laughter. It's Paul Ryan. To the dozens gathered, Ryan is a hometown hero. He's the boy wonder who emerged from Janesville, population 63,820, to become...