While super PACs have received plenty of attention since the Citizens United ruling for flooding money into elections, a new investigative report shows how social welfare groups known as 501(c)(4)s "are pouring much of their resources" into elections.

The report released Sunday from Kim Barker of ProPublica, How Nonprofits Spend Millions on Elections and Call it Public Welfare details how nonprofit groups known as 501(c)(4)s are outspending all TV advertising in the presidential race -- surpassing that of super PACs.

These nonprofit groups, such as the Republican Jewish Coaltion or Americans for Prosperity, get tax exempt status "for the promotion of social welfare," but

[ProPublica's] examination shows that dozens of these groups do little or nothing to justify the subsidies they receive from taxpayers. Instead, they are pouring much of their resources, directly or indirectly, into political races at the local, state and federal level.

The 2010 election functioned, effectively, as a dry run, providing a blueprint for what social welfare groups are doing on a larger scale today.  

While super PACs must disclose their donors, donors to the 501(c)(4) groups remain behind a veil of secrecy.  "Like super PACs, they can rake in unlimited contributions, support and oppose candidates, and buy ads right up until Election Day. But unlike super PACs, they don't have to disclose their donors," writes Barker.

Barker also explains how the Citizens United ruling helped foster this situation:

Previously, laws had barred nonprofits from accepting donations from corporations or unions for political purposes and had mostly restricted 501(c)(4)s to generic "issue" ads that stopped short of calling on people to vote for or against candidates.

Citizens United dismantled this system. In a 5-4 decision, the high court said corporations and unions enjoyed the free speech rights of any individual. They could spend directly on political ads or give unlimited amounts of money to nonprofits for political activities. Over the next two years, contributions to existing social welfare nonprofits skyrocketed and new ones geared specifically toward elections were formed.

"It really sounded the starting gun for the creation of nonprofits that were strictly political in nature," said Sheila Krumholz, executive director of the Center for Responsive Politics, a nonpartisan research group that tracks money in politics.

Click here to read the full story.