Overall, Rivers and Geer found, positive ads from either side didn't have a measurably significant effect on voters. Negative ads about Obama didn't have much of an effect either. The only commercials that seemed to have a clear effect were negative ads about Romney.
Still, the candidates are also airing a few positive ads touting their achievements or rebutting negative ads — but very few so far. And the negative ads won't come down soon, because as Geer and others have noted, the messages of political ads wear off quickly unless the campaigns hammer away at them, week in and week out.
As Obama once said during his bare-knuckled primary battle with Hillary Rodham Clinton: "I'm from Chicago. We don't play in Chicago. We take our politics seriously."
The Obama onslaught has prompted plenty of hand-wringing in Republican ranks. Some GOP strategists have publicly questioned why the Romney campaign has failed to air more positive information about the candidate — more of his impressive biography, more commercials showing his photogenic family, more details about his economic plans.
Romney says he thinks he still has plenty of time for that. "This is still early for a lot of people in the political process," he told NBC News last week. "Labor Day is usually the time when people focus more attention on candidates. Most folks won't really get to see me until the debates."
But the first debate won't come until Oct. 3, more than two months from now. Romney doesn't really mean to wait that long; the Republican convention at the end of August will be one long Romneyfest. Still, every week he delays in revealing more of his himself to the electorate, the fewer undecided voters may be left to listen.
Doyle McManus is a columnist for The Los Angeles Times.