Michael Scherer explains it:
What Romney must fear is doing anything that weakens his case that he is the strongest person to take on Obama. If he gets ruffled, if he misspeaks, if he snaps at one of his opponents, then he will have reestablished himself at the center of his campaign, and thereby stepped on his own strategy. Romney’s job tonight is to deliver his message, challenge Obama and get out of his own way. He is already running as if he’s in the general election, and unless his rivals can knock him off his game, the Romney strategy has a good chance of getting him there.