David Brock super PAC provides cover for its donors and Obama

Super PACs can raise and spend unlimited amounts but are barred from coordinating their activity with the candidates they support or with the national political parties.

They can work with other outside groups. That's what American Bridge does each day, fact-checking commercials and sharing its research with other liberal groups, including Priorities USA Action, a pro-Obama super PAC.

"It's a lot more efficient," said Shauna Daly, American Bridge's research director who was deputy research director in Obama's 2008 campaign. "If you want to look through all of the state Senate votes of a candidate, it's best to look through it once and share what we've found with your friends, rather than have five groups do the same research.