NPR examined the finances of the superPAC American Crossroads and the 501(c)(4) Crossroads GPS.
When the groups started out two years ago, donors gave almost evenly to both. But since then, the donors have flocked to the secret side. In the first three months of this year, nearly 80 percent of the incoming cash went to Crossroads GPS.
And it wasn't coming from ordinary campaign donors. In 2010 and 2011, nearly 90 percent of the Crossroads GPS money came in chunks of a million dollars or more. Most of this money ends up on TV.
Dan Backer is a campaign finance lawyer whose client base includes candidates, superPACs and 501(c)(4)s, mostly on the right.
"A lot of the communications from the superPACs and from the '(c)(4)s' and the campaigns are essentially interchangeable. If you didn't have outside groups, I think campaigns would be running a lot of these same ads," says Backer.
One legal hurdle is that the Internal Revenue Service says 501(c)(4)s cannot intervene in political campaigns as their primary activity.
Now here's the loophole: If an ad doesn't tell voters how to vote, it can count as an issue ad — not a political one.
So 501(c)(4) ads use tag lines like: "Tell President Obama American workers aren't pawns in your political games," which comes from Americans for Prosperity.
"If you spend 25 seconds bashing a candidate or bashing a position he holds, and then say, 'call this candidate up and tell him not to do this bad thing that they're doing,' you know, it's a lawyer's line that they're setting up. So at the end of the day, there really is no difference in my book," says Adam Strasberg, a media consultant on the left who has produced plenty of issue ads.