The Obama campaign has dominated the airwaves with ads in key battleground states in the last three weeks, contrary to expectations that the war chest of Republican-friendly Super Pacs would eclipse Democratic spending, according to a new study.

The president's campaign and his supporters have have topped all other spenders in the presidential race since the national conventions, outspending the Romney camp by two to one, the report found.

In the key swing states of Virginia, Ohio and Florida, pro-Obama ads outnumbered pro-Romney ads by as much as 50% in 14 of all 15 of the biggest media markets. In the same 15 markets, pro-Romney advertising outpaced Obama only in Las Vegas, Nevada, according to the Wesleyan Media Project, which based its report on Kantar Media/CMAG data.

Obama and his allies aired 1,800 more ads than Romney and his allies in Denver; 1,700 more in Norfolk, Virginia, and 1,500 more in Orlando – all in the three weeks from 9 to 30 September.

The report's authors said they believed Obama's advertising advantage may explain why polls in key states have shifted his way in recent weeks.

Erika Franklin Fowler, the co-director of the Wesleyan Media Project, which carried out the study, said: "The heavy advertising from the Obama campaign has challenged the assumption that Romney-friendly outside groups would saturate and dominate the airwaves in key markets.

"The heavy Obama advantage may be one reason why polling in battleground states has moved against Romney in recent weeks."

While Obama spenders topped all others in the presidential race, the Romney campaign relied heavily on outside groups to sponsor his advertising. One reason Romney held an ad advantage in Vegas was because outside groups sponsored over 2,900 ads, more than double those paid for by the Romney campaign in that media market in the last three weeks.

Almost half of the pro-Obama ads mentioned taxes, while 93% of the pro-Romney ads spoke of jobs, compared to one-in-three pro-Obama ads which mentioned jobs. The report found that not only is 2012 the most negative campaign in recent history, it has also featured an overwhelming number of appeals to anger. In an analysis of the "top emotional appears" the report listed a "strong" or "some" appeal to anger as driving 93% of pro-Obama ads, and 69% of pro-Romney ads.

"This year, pro-Obama spots are more likely to use pure attacks than pro-Romney spots, and pro-Romney spots are slightly more likely than pro-McCain ads were in 2008 to use pure attacks (and much more likely to use contrast spots than McCain)," it said.

In Florida, pro-Obama ads have accused Romney of outsourcing jobs to China, trying to gut Medicare and failing to release his tax returns.

Travis Ridout, co-director of the Wesleyan Media Project, said: "Anger was the most common emotional appeal made in presidential advertising in the last three weeks, and that was true for Obama and Romney ads."

The Democratic national committee was not involved in any of the ads, while the Republican national committee has had limited involvement.

Michael Franz, co-director of the project, said that may be because the DNC was concentrating on other areas of its base, such as voter contact drives. He said: "The RNC coordinated on a significant amount of advertising with Romney and independently supported him as well, throughout the first part of the general election. The Republican party may have shifted its focus to congressional ads in the fall campaign, or may be conserving resources for a late push."

Both the Democratic and Republican candidates are airing more ads and spending more on them this year than did their counterparts in 2008, it found. However, Republican ads are proportionally higher. Ad spending supporting the Democratic candidate is up 20% over 2008, while ad dollars supporting the Republican are up 93% over the 2008 level.

Obama had significant advantages both times so far, though the gap between Romney and Obama is smaller than that between McCain and Obama, it said.

The Wesleyan Media Project was set up in 2010 in the wake of the Citizens United ruling from the US supreme court that opened the floodgates to a vast infusion of corporate and trade union money into the political process. The project aims to inform the public about the growing influence of special interests in political campaigns by tracking the proliferation of 30-second TV attack advertising in federal elections.

Using data provided by Kanta Media/CMAG, the Wesleyan team can monitor when, where and how frequently political ads are being broadcast across the 210 US media markets. By analysing the content of each advert, it also identifies which candidate, party or outside special interest was responsible for it. © Guardian News and Media 2012