Voters expect politicians to stretch the truth. But when the offender is as persistent with mistruths, half-truths and no-truths as Mr. Obama is, voters expect the other candidate to blow the whistle. They want their leaders to show toughness and be competitive. Which brings us back to the coming Oct. 3 debate, to be followed by two others on Oct. 16 and 22.

During these widely watched events, Mr. Romney must call out the president. That is not so easy: Mr. Romney can’t call Mr. Obama a liar; that’s too harsh a word that would backfire. Mr. Romney must instead set the record straight in a presidential tone—firm, respectful, but not deferential. And a dash of humor is worth its weight in gold. …

By carefully calling into question the president’s veracity, Mr. Romney will have the opportunity to provide context: Mr. Obama doesn’t shoot straight because he can’t defend his record and has no agenda for the future except the status quo, stay the course.