CLEVELAND — If you switched on a television in northern Ohio this week, chances are you saw an ad for President Obama’s reelection campaign. Or two. Or three. Over and over again. Mitt Romney? Nothing. Obama’s Republican challenger has spent much of the summer racing around the country gathering campaign donations to pay for advertising. And the legal restraints that kept him from spending money raised for the general election were lifted a week ago, when the party officially nominated him for president. Yet for reasons that his advisors declined to discuss, Romney has ceded the advertising airwaves to Obama...