With Mitt Romney facing the biggest challenge to his presidential aspirations since he announced his candidacy, his aides acknowledged Friday what seemed unthinkable just seven days ago: He could lose the South Carolina primary. After arriving here last week fresh off of what seemed to be two victories in a row in Iowa and New Hampshire, Mr. Romney was suddenly confronting the prospect of leaving as the winner of only one of the first three nominating contests. Having been stripped of his victory in Iowa on Thursday after a recount that gave the state to Rick Santorum, Mr. Romney now...