FLORENCE, S.C. — Mitt Romney delivered a 13-minute speech here on Tuesday morning, shook hands with an audience that filled only a small part of a ballroom and caught a plane for a fund-raising gala in New York, a clear sign that he is looking beyond South Carolina. The outward confidence in Mr. Romney’s posture belied a deeply uncertain and fluid conclusion to the Republican presidential primary here. His rivals have shifted away from assailing his business record and are reminding conservatives through radio and television advertisements about the health care bill he signed as governor of Massachusetts and about...