As important as super PACs have been in the 2012 election, the loosening of political spending rules for non-disclosing, nonprofit organizations was also a key development after the Citizens United decision.

Republican-aligned nonprofits have outspent their Democratic counterparts by more than 8 to 1.

Federal law requires nonprofits’ spending to be reported only if a group’s advertisements encourage viewers to vote for or against a candidate, or if they mention a candidate shortly before a political convention or election.

Justice Anthony Kennedy, the author of the Supreme Court’s Citizens United 5-4 opinion, made a point of saying that disclosure was a key part of the court’s rationale. Disclosure would allow citizens to monitor the new political activity.

"This transparency enables the electorate to make informed decisions and give proper weight to different speakers and messages," he wrote.

But the tax-exempt groups are spared by Internal Revenue Service and FEC rules from having to publicly reveal their donors.

Researchers Robert Maguire of the Center for Responsive Politics and Alexandra Duszak of the Center for Public Integrity contributed to this report.