If the Republican primary is now a Mitt Romney-vs.-Herman Cain contest, Romney has an ace up his sleeve: One of his top advisers has run a winning campaign against Cain before. Not many in politics can make that claim. In his only real previous try for elected office, Cain ran in Georgia's 2004 GOP Senate primary, coming in second to now-Sen. Johnny Isakson. Back then, Romney strategist Stuart Stevens was a consultant to Isakson. Of Cain, Stevens recalls: "He scared the heck out of us." Cain's 2004 candidacy positioned him to the right of an establishment candidate distrusted by the...