Nate Silver measures the impact of campaign advertising.
Campaign ads matter more when a candidate can outspend the opponent. This simple fact sometimes gets lost because people fixate on the content of ads. But the volume of ads may matter more. Consider the 2000 presidential election. In the final two weeks of the campaign, residents in battleground state were twice as likely to see a Bush ad as a Gore ad. This cost Gore 4 points among uncommitted voters. The same thing happened in 2008, when Mr. Obama vastly outspent his Republican opponent, Senator John McCain.