* Outside groups funded more than half of pro-Romney ads
* Presidential ads notably negative compared to 2008
* U.S. Supreme Court ruling paved way for Super Pacs
By Alina Selyukh
WASHINGTON, Sept 12 (Reuters) - Unlimited spending driven by Republican groups is responsible for an outsized share of advertising in the 2012 campaign season that feeds the markedly negative stream of ads, according to an academic analysis released on Wednesday.
Super PACs, or political action committees, and tax-exempt advocacy groups accounted for nearly a third of all the ads aired in the U.S. presidential race, according to a study by the Wesleyan Media Project that analyzed broadcast and national cable spots run between April 26 and Sept. 8.
"The key dynamic of this campaign is the increased presence of these outside groups in all key races across the federal landscape," said Michael Franz, co-director of the project and associate professor of government at Bowdoin College in Maine.
Republican outside groups are largely responsible for this year's trend. Of 302,580 ads backing Republican presidential candidate Mitt Romney, outside groups funded 54 percent, spending $117.5 million.
Meanwhile, the official Romney campaign spent $37.8 million for 30 percent of the pro-Romney ads.
"We've never seen that big of a share of outside group spending in presidential races before," Franz said.
Democrats have been unable to match the fundraising prowess of these unlimited-funding groups. President Barack Obama's re-election campaign paid $123.4 million for 89 percent of a total of 315,556 ads backing him, while groups helped with 9 percent of those ads, investing $14.1 million.
In 2008, outside groups funded 2 percent of pro-Obama ads and less than 4 percent of ads supporting his Republican rival, John McCain, according to the study.
In 2010, the U.S. Supreme Court in its landmark "Citizens United" ruling removed limits on corporate and union spending in elections, paving the way for Super PACs and tax-exempt advocacy groups to raise and spend unlimited funds this election as long they do not coordinate their efforts with official campaigns.
NEGATIVE AD SEASON
This year's presidential election is also stacking up to be more negative than the previous one, according to the study that found both parties more willing to employ attack ads.
More than 72 percent of pro-Romney advertising attacked Obama and 46 percent of pro-Obama ads returned fire. In 2008, less than a quarter of pro-Obama and 41 percent of pro-McCain spots were negative.
"Because of all that group involvement, we're seeing a lot more negativity," said Travis Ridout, also a co-director and associate professor of political science at Washington State University. "When the outside groups are advertising, we're finding that they're predominantly negative."
But the campaigns themselves are also not shying away. More than four in ten Obama campaign ads and more than half of Romney campaign ads were negative.
The top issue featured in spots on both side of the aisle was jobs, but pro-Romney spots were three times as likely to mention the federal budget deficit than Democratic ads, the study found. Pro-Obama ads were more likely to talk about taxes.
Pro-Republican groups also accounted for nearly a third of all ads since June 1 backing party candidates running for seats in the U.S. Senate and House of Representatives. Pro-Democratic groups sponsored about a quarter of ads backing their party candidates.
That's notably less than 2008 and even the mid-term election of 2010 when all attention was on brutal House races.
Crossroads GPS, a nonprofit advocacy group founded by former George W. Bush aide Karl Rove, was the biggest outside spender in House and Senate races from June 1 through Sept. 8.
The Chamber of Commerce, a tax-exempt business lobby that largely backs Republican candidates, came in second, followed by Democratic Majority PAC, a Super PAC.
The Wesleyan Media Project tracks and analyzes political television advertising in real-time based on data provided by ad tracking firm Kantar Media/CMAG.