Citizens United didn't actually change anything about what these groups can do. They could spend unlimited amounts during election season without disclosing their donors before this past January.
What Citizens United did change was where their money can come from.
The Supreme Court ruled in Citizens United that corporations and unions can spend directly on elections. Previously, they were prohibited, and these "shadowy" nonprofits weren't allowed to take corporate money, otherwise they'd have to disclose their donors to the Federal Election Commission. Now, after Citizens United, 501(c)4 groups can take as much corporate and union money as they want, still without disclosing.
But while Democrats have insinuated that Citizens United is preventing these groups from having to disclose their donors, that simply isn't the case. 501(c)4 groups never had to disclose their donors.
"Citizens United did not change the disclosure laws at all. It endorsed the disclosure laws on the books, and the weaknesses that are being blown into huge proportions in this election were on the books before this election," said Tara Malloy, an attorney with the Campaign Legal Center. The Federal Election Commission, meanwhile, has weakened those laws through its interpretations, Malloy said.
In fact, the Supreme Court actually came down in favor of disclosure in one key regard.
There has been a hanging question in campaign finance law for some time over just what constitutes a political communication. For hypothetical instance, an ad is aired in California two weeks before Election Day. In it, a narrator advises viewers, "Sen. Barbara Boxer is so mean to poor children it's shocking. Call Sen. Barbara Boxer and tell her to stop saying mean things to poor children and taking their money to buy extra hot tubs for her diamond-encrusted hover-yacht." Does that count as an "electioneering communication"?
According to some past interpretations of the law, it doesn't, because the ad doesn't expressly advocate for Boxer's election or defeat. The narrator just wants you to call Sen. Boxer and ask her not to be so horrible--he's not telling you to vote against her.
But in Citizens United, the Supreme Court cited and reinforced a more liberal interpretation: that mentioning a candidate by name close enough to Election Day, even without expressly telling viewers how to vote, can count as an electioneering communication and thus require disclosure.
Unsurprisingly, it gets even more complicated.
A group has to tell the FEC that it's airing the ads and how much it's spending, but it still doesn't have to disclose its donors. In order to keep the donors secret, the group simply has to file with the IRS as a tax-exempt nonprofit, claiming that its main function is not politics, instead of openly forming as a political committee.