Republican John McCain holds a steady 7-point lead over rival Mike Huckabee in South Carolina two days before the state casts votes in an unpredictable presidential nominating race, according to a Reuters/C-SPAN/Zogby poll released on Thursday.Support for McCain, an Arizona senator, held firm overnight at 29 percent, with former Arkansas Gov. Mike Huckabee sliding one point to 22 percent. Former Tennessee Sen. Fred Thompson climbed two points to move into third place at 14 percent. A portion of the polling was conducted after former Massachusetts Gov. Mitt Romney's breakthrough win in Michigan on Tuesday, but he still slipped one point to 12 percent. Texas Rep. Ron Paul and former New York Mayor Rudy Giuliani were tied at 5 percent.