In Senate races, Democrats are fighting to preserve their thin majority. Their party campaign committee wants the Federal Election Commission to crack down on some of the Republicans' wealthiest allies — outside money groups that are using anonymous contributions to finance a multimillion-dollar onslaught of attack ads.

At the Democratic Senatorial Campaign Committee, director Matt Canter says the pro-Republican groups aren't playing by the rules. The committee plans to file a complaint with the FEC accusing a trio of "social welfare" groups of actually being political committees, abusing the rules to hide the identities of their donors.

"These are organizations that are allowing right-wing billionaires and corporations to essentially get special treatment," says Canter.

Democrats don't have high-roller groups like these. Canter says that while ordinary donors in politics have to disclose their contributions, "these right-wing billionaires and corporations that are likely behind the ads that these organizations are running don't have to adhere to any of those laws."

The complaint cites Crossroads GPS, co-founded by Republican strategist Karl Rove; Americans For Prosperity, supported by the billionaire industrialists David and Charles Koch; and 60 Plus, which bills itself as the senior citizens' conservative alternative to AARP.

The three groups have all told the IRS they are social welfare organizations, just like thousands of local civic groups and definitely not political committees.

Canter said they've collectively spent about $22 million attacking Democrats in Senate races this cycle.

The Obama campaign filed a similar complaint against Crossroads GPS last month. Watchdog groups have also repeatedly complained to the FEC and IRS.

At Crossroads GPS, spokesman Jonathan Collegio said their ads talk about things like unemployment and government overspending. "Those are all issues and advertising that's protected by the First Amendment, and it would ... be de facto censorship for the government to stop that type of advocacy from taking place," says Collegio.

And on Fox News recently, Rove said the Crossroads organization is prepared to defend itself and its donors' anonymity.

"We have some of the best lawyers in the country, both on the tax side and on the political side, political election law, to make certain that we never get close to the line that would push us into making GPS a political group as opposed to a social welfare organization," says Rove.

But it's possible that the legal ground may be shifting slowly beneath the social welfare organizations.

They've been a political vehicle of choice for big donors who want to stay private, especially as the Supreme Court loosened the rules for unlimited money.

But last month, a federal appeals court in Richmond, Va., said the FEC has the power to tell a social welfare organization that it's advertising like a political committee and it has to play by those rules.

Campaign finance lawyer Larry Noble used to be the FEC's chief counsel. He says that court ruling won't put anyone out of business this year.

"But it will have a chilling effect on these groups of billionaire-raised contributions, because it will call into question whether or not they're really going to be able to keep their donors confidential," says Noble.

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