Democrat Barack Obama's big lead over rival Hillary Clinton has slipped but is still substantial two days before the presidential primary election in South Carolina, where candidates were competing vigorously to win support from black voters.Former President Bill Clinton, whose wife is battling with challenger John Edwards to stay out of third place in South Carolina, lashed out at Obama and the media for elevating race in the campaign. Obama's lead fell 3 points overnight to give him a 39 percent to 24 percent edge over Clinton in South Carolina, according to a Reuters/C-SPAN/Zogby poll released on Thursday.