Campaign ads want viewers to think they're just exposing the truth that voters need to hear, but a new study finds campaign attack ads from outside groups are about 85 percent false.

Analysis conducted by the Annenberg Public Policy Center on behalf of the Center for Responsive Politics indicates that four of the biggest third-party spenders in political advertising have not only been shelling out the big bucks on attack ads that contain false claims over the past six months. The vast majority of spending - 85 percent - by these groups, referred to as 501(c)(4)s because of their place in the tax code, was on ad spots that included at least one false assertion.

The ads were fact-checked by the groups like FactCheck.org, PolitiFact.org, the Washington Post's Fact Checker and the Associated Press.

According to the IRS, a 501(c)(4) is an organization meant to "promote social welfare to benefit the community."

In this case the community seems to support the GOP. Each of the four top-spending groups named in the report lean Republican. As of June 1, no ads had been sponsored by a 501(c)(4) with Democratic leanings.

Crossroads GPS, the group founded by Karl Rove, has the worst record, according to Annenberg. It has dropped $10.3 million on attack ads with at least one inaccuracy. The organization is the creator of many anti-Obama ads. The most recent being "Stopwatch," which claims that Obama's negligence is growing the national debt by $1.4 million every 30 seconds.

In second place is The American Future Fund, which has spent $6.4 million on ads with at least one false claim, according to the fact checkers. It has also spearheaded the attempt to convince the FEC to loosen the definition of "clearly identified candidate" in order to sidestep regulations requiring organizations that run ads attacking candidates to reveal their donors. Read more about that effort here. Americans for Prosperity and the American Energy Alliance, the third- and fourth-ranked spenders, have shelled out more than $5 million and $3.3 million, respectively, for ads with errors.

None of the groups ranked by Annenberg, thanks to their 501(c)(4)s status, are required to disclose their donors.

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