"The ad wars" is a series Wonkblog will be running semi-regularly until the election, in which George Washington University political scientist (and Monkey Cage founder) John Sides shows you what's really happening on the airwaves.

The Washington Post has a neat feature displaying the presidential election’s political advertising — who is airing it, where, and how much. Using these advertising data, which come from CMAG|Kantar Media, I want to take stock of the the ads, starting back in January and continuing through Sept. 9.

What this analysis reveals: Just prior to the Republican National Convention, Mitt Romney, the Republican Party, and its affiliated independent groups began to open up a large advantage in televised advertising. But during the RNC and subsequently, the Republicans advertised much less, returning the advantage to Obama.

To measure advertising, I am going to deviate from the usual practice of looking at dollars spent. This is because spending buys different amounts of advertising in different media markets, some of which are more expensive than others. Instead, I will look at the actual number of "spots" or individual airings of ads. This gives us a more realistic sense of what voters in battleground states may have been exposed to, depending on how and how much they watch television.

To begin, here is the total number of ads aired in the presidential general election by every entity — Obama, Romney, both political parties and all the various independent groups:

This shows that ads sponsored by Obama, the DNC and Democratic-affiliated groups outnumbered those aired by their Republican counterparts up until the end of July. Then, with the exception of the second week of the Olympics, Republican ads began to outnumber Democratic ads by 10,000 or more, but this advantage disappeared during the weeks of the RNC and DNC — as the Republicans stood down while Democrats maintained comparable levels of advertising.

This pattern was evident regardless of which Republican entity — Romney's campaign, the Republican National Committee or an affiliated group — sponsored the ad. Here are the trends broken down by sponsor: candidates, candidates in coordination with the parties, the parties, and independent groups.

In general, Obama has consistently out-advertised Romney, but his advantage has been mitigated or even eliminated by the efforts of independent groups on the Republican side and, to a lesser extent, the Republican National Committee. Over this period, approximately 82 percent of the ads aired by independent groups on the Republican side came from three groups: Restore our Future (19 percent), Americans for Prosperity (23 percent)  and Crossroads GPS (40 percent). This is a dramatic departure from 2008.  As the Wesleyan Media Project notes:

... since the end of April, pro-Romney or anti-Obama groups have sponsored 54 percent of Republican ads.  By comparison, through the 8th of September in 2008, pro-McCain or anti-Obama groups accounted for only 3.5 percent of Republican ads.

However, because both Romney and Republican-affiliated independent groups chose to advertise less these past two weeks, Obama's ads have dominated.
This raises an interesting possibility, notes Wesleyan political scientist Erika Franklin Fowler:

During both the Democratic and Republican conventions, ads favoring Obama dominated the airwaves in numerous markets, including key swing states such as Colorado, Ohio, Nevada, and Virginia.  This advantage may help to explain why Obama’s ‘convention bounce’ was larger than Romney’s.

What makes this hypothesis plausible is that imbalances in advertising spending are often necessary for advertising to influence the polls.  If the candidates are at parity, then the two sides' spending may simply cancel each other out.  And, as some new research suggests, these imbalances may be more prevalent in presidential campaigns than previously thought.  Certainly a lack of complete parity has been the norm in each of the past several months.

But Obama supporters should not get too excited.  The effects of ads also decay quickly.  Any advantage over these past several months, or even these past two weeks, should be less important than an advantage in late October.

John Sides is an Associate Professor of Political Science at George Washington University. He specializes in public opinion, voting, and American elections.