Shortly after Barack Obama won reelection in November, New Jersey governor Chris Christie pointed out that Republicans’ cloudy political prospects had a bright silver lining. “One of the reasons you have 30 Republican governors in America, and why we’re the only organization to add Republican strength,” Christie said, “is because people see us getting things done.” Christie’s stance countered most of the elite postelection commentary, which gleefully pronounced the Republican Party’s political irrelevance. But the governor was right. Since Obama first took office in 2008, Republicans have picked up a net nine governorships, bringing their total to 30 states,...