How big an impact did Mitt Romney's performance in last week's debate have? Huge. Mr. Romney not only won the night, he changed the arc of the election—and perhaps its outcome. Surveys have him leading the RealClearPolitics average of polls for the first time since securing the GOP nomination in mid-April.

Prior to Oct. 3, Mr. Romney trailed President Barack Obama by an average of 3.1 points in national polls tallied by RealClearPolitics. Since the debate, Mr. Romney now leads...