* Republican Super PACs help Romney bridge gap
* Obama making gains in polls in several key states
By Alina Selyukh
WASHINGTON, Oct 3 (Reuters) - Ads for President Barack Obama's reelection campaign dominated TV airwaves in key states in September, despite Republican challenger Mitt Romney's powerful "Super PAC" backers, according to an academic analysis released on Wednesday.
Republican Super Political Action Committees and tax-exempt advocacy groups - which can raise and spend unlimited funds - had been widely expected to give Romney an advantage in the campaign for the Nov. 6 election, as they focus almost exclusively on advertising, a crucial way to reach voters.
However, the new study by the Wesleyan Media Project found that the Obama campaign, not including spending by the weaker Democratic outside-financing groups, has aired more ads in swing states than Romney and his top outside backers together.
The study found that from April 25 to Sept. 30, the Obama campaign spent $163.8 million on 363,010 ad airings, while the Romney campaign spent $57.3 million on 126,748 ads.
Top Republican outside financing groups, which have to pay more for ads than candidates themselves, helped Romney bridge the gap - though not completely - spending another $129.2 million on a total 181,578 ads.
The study, based on data provided by ad-tracking firm Kantar Media/CMAG, analyzed broadcast and national cable spots run from Sept. 9-30, as well as totals run since late April, when Romney first emerged as the likely Republican nominee.
Obama has made gains against Romney in the polls in several key battleground states and the Wesleyan project co-director Erika Franklin Fowler said advertisements may have helped.
Obama and his allies have aired more ads than Romney and his backers in 14 out of 15 media markets attracting the most political advertising in the past three weeks, including in the key states of Colorado, Ohio, Virginia and Florida.
Republicans aired more ads since Sept. 9 in only one place, Las Vegas, the market seeing the second-highest number of ads.
Other top saturated markets in September were Denver, Cleveland and Tampa, the analysis showed.
The top outside spenders helping the Republicans were the tax-exempt non-profit Crossroads GPS and its Super PAC sister American Crossroads, both run by Karl Rove, a former aide to President George W. Bush; Americans for Prosperity, backed by conservative billionaire Koch brothers; and Restore Our Future, a Super PAC devoted purely to electing Romney.
The only outside Democratic group among the top ten spenders was Priorities USA Action, a Super PAC run by former Obama aides. Since late April, it has spent $14.6 million on a total of 31,707 ad airings.
The Republican National Committee has also helped Romney with 49,920 ads, spending $33.6 million up until the end of September. The Democratic National Committee, together with Obama, spent $15.3 million on 7,210 advertisements.
(Reporting by Alina Selyukh; Editing by David Brunnstrom)
Keywords: USA CAMPAIGN/MONEY ADS