More than two thirds of all ads aired in the presidential race so far have been attacks, a drastic increase compared to recent presidential campaigns, according to a new study by the political ad-tracking Wesleyan Media Project that covers the race from the start of 2011 through April 22 of this year.

Of the ads aired during that period, 70 percent were negative, compared to 10 percent in the last presidential campaign. Political scientist Michael Franz, a co-director of the project, called the change "absolutely astonishing."

The negative tone is being driven by outside groups, responsible for 86 percent of the negative ads the study found. Those interest groups include super PACs, products of two landmark court cases in 2010 that can raise and spend unlimited funds to promote or bash candidates. With six months to go before the election, super PACs already have raised more than $200 million. Another recent study of political ads by the Annenberg Public Policy Center found that super PACs are responsible for a super majority of misleading advertisements.

"One reason the campaign has been so negative is the skyrocketing involvement of interest groups, who have increased their activity by 1100 percent over four years ago," political scientist Erika Franklin Fowler, a co-director of the project, said in a press release. But she added that candidates' own ads have also become nastier.

In another shift from prior years, outside interest groups–mostly super PACs–have become the dominant ad buyers. In the presidential race, 60 percent of ads have come from these groups—up from three percent in 2008, according to the study. Franz called the jump "truly historic."

"Usually an interest group will become involved in lower ticket races at high levels like that," he said. Traditionally, he said, interest groups stay on the sidelines during the presidential nominating contests.

The closest comparison to this year's level of activity by outside groups, he said, is 2004, when about one quarter of general election ads targeting Democratic candidate John Kerry were from interest groups, led by Swift Boat Veterans for Truth and MoveOn.org.

The Wesleyan Media Project gets its data from Kantar Media/CMAG, a company that sells the ad data. The Wesleyan study found that most of the ads focused on the GOP presidential nominating contest came from the candidates themselves or from super PACS, which have to report their donors to the FEC. But the authors also found that a vast majority of the general election ads—most of which have criticized President Obama—are from groups that do not disclose their donors—nonprofit organizations organized as 501c4s.

(MORE: Many political ads fly under the campaign reporting radar)

In fact, four of the top five outside groups playing in the general election are shadowy nonprofits: The Republican-leaning Crossroads GPS, Americans for Prosperity, the American Energy Alliance and American Future Fund. In all, the study finds that just over $40 million has been spent by outside groups, candidates and party committees for the general election, when looking at the top 12 spenders.

The study does not provide detailed numbers of spending on congressional and state races but does offer an April snapshot. The outside groups that placed the most ads from April 1 through April 22 were:

• Club for Growth: Indiana and Nebraska Senate primaries
• Right Direction Wisconsin PAC: Wisconsin governor recall election
• Americans for Prosperity: Nebraska Senate
• American Action Network: Florida and Indiana Senate
• League of Conservation Voters: Ohio Senate and Pennsylvania's 17th Congressional District
• Citizens for Strength and Security: Montana Senate
• America Works USA: Missouri governor
• Patriot Majority USA: Missouri Senate, Georgia's 12th Congressional District
• Constitution Trust: North Carolina lieutenant governor
• Campaign for Primary Accountability: Pennsylvania's 17th and 18th Congressional Districts

Overall, the study estimates that $112 million has been spent on ads for the presidential race this cycle. That's down from $190 million in 2008 for the same period, and the authors say that's because only the Republican presidential nomination was contested this year. Both parties had wide open races in 2008.

The study's authors note a couple of caveats to the data–it covers broadcast TV and national cable buys, not local cable. They also note that the costs they detail are estimates. The only way to get precise figures for campaign ad spending is to extract them from files that the Federal Communications Commission requires each television station to make available to the public. The Sunlight Foundation, along with the Free Press and other journalism and open-government partners, is involved in an effort to recruit volunteers to do that and build a searchable sortable database in time for this year's election.