Mitt Romney keeps his South Carolina headquarters in a single-story building at one end of Gervais Street, which is Columbia’s version of Washington’s K Street, lined with the offices of local lobbyists, P.R. consultants, and other fixers and power brokers. The main room of Romney HQ is decorated with hand-painted red-and-blue signs, mementos from previous campaign events: mitt is my hero! mitt’s the man! mitt’s my pick! I visited on a steamy Monday night in late July, a time of year when few South Carolinians are interested in politics and fewer still want their evening interrupted by pitches for a...