GREER, South Carolina _ In a presidential primary state unmatched for its history of racial politics, Mitt Romney arrived with the dumb luck of seeming far less venomous than some other contenders. Rivals Newt Gingrich and Rick Santorum bit themselves in Iowa and New Hampshire with stereotyped associations of African Americans with food stamps, while Ron Paul was embarrassed by a 1992 newsletter in which he said the Los Angeles riots over the Rodney King verdict ended only when “it came time for the blacks to pick up their welfare checks.’’