Throughout the 2012 election cycle, Sunlight followed the unlimited money. From super PACs and corporations to unions and “dark money” we collected, in real time, the political spending reported by these outside groups.

With the 501(c)(4) social welfare nonprofits back in the news again (and the IRS’s enforcement of them), we wanted to take a closer look at how these organizations spent money to influence the 2012 election. We often use the term “dark money” to describe these groups since they can spend an unlimited amount on independent expenditures and electioneering communications yet they do not have to disclose their donors. For more information on how to track all types of federal campaign finance disclosures, check out this handy infographic.

Overall, dark money groups reported $300 million in independent expenditures in 2012. Of the 50 groups who spent the most, 15 are 501(c)(4) nonprofits. Using our Follow The Unlimited Money tracker, Political Ad Sleuth, Ad Hawk and return on investment calculations, here is how they made an impact in the race for the White House and Congress.

Please note that Americans for Prosperity spent $33.5 million overall in 2012 focusing on opposing just a single candidate: Barack Obama. It is separated from the list below since the others spent money in more than one race. Figures below are also reflective of just general election spending.

Crossroads GPS

This conservative group spent more than $70.5 million and had a return of investment of 14.42 percent. More than $15 million was spent to oppose Barack Obama and more than $10 million was spent to oppose now Virginia Senator Tim Kaine. Watch ads and review ad files.

American Future Fund

This conservative group spent nearly $24 million and had a return of investment of 5.57 percent. More than $11 million was spent to support Mitt Romney. Watch ads and review ad files.

Americans for Tax Reform

This conservative group spent more than $15.7 million and had a return of investment of 57.35 percent. The group only spent in congressional races. Review ad files.

American Action Network Inc.

This conservative group spent more than $11.7 million and had a return of investment of 60.33 percent. The group only spent in congressional races. Watch ads.

League of Conservation Voters Inc.

This liberal group spent $10.3 million and had a return of investment of 83 percent. It spent $2 million to oppose Virginia GOP Senate candidate George Allen and $957,000 to oppose Mitt Romney. Watch ads.

Review the remaining 10 top spending 501(c)(4) nonprofits in this spreadsheet or below.

Visit the Sunlight Reporting Group for a review of the role political nonprofits have played in recent elections.