In a bitter campaign here infused with discussions of race, Mr. Obama’s convincing victory puts him on equal footing with Mrs. Clinton — with two wins each in early-voting states — and gives him fresh momentum as the contest plunges into a nationwide battle over the next 10 days.
Former Senator John Edwards, a native of South Carolina who was trying to revive his candidacy, came in third place but vowed to keep his campaign alive, despite failing to win a single state so far.
With 99 percent of the electoral precincts reporting, Mr. Obama had 55 percent of the vote, Mrs. Clinton had 27 percent, and Mr. Edwards had 18 percent.
“Tonight, the cynics who believed that what began in the snows of Iowa was just an illusion were told a different story by the good people of South Carolina,” Mr. Obama told a euphoric crowd here after the results came in. “After four great contests in every corner of this country, we have the most votes, the most delegates and the most diverse coalition of Americans we’ve seen in a long, long time.”