Sen. Jeanne Shaheen (D-N.H.) in the U.S. Capitol building May 14, 2013. Groups are creating ads in New Hampshire to attack Shaheen 10 months before the midterm congressional elections. Allison Shelley/Getty Images hide caption

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A composite image shows part of the NPR/Center for Responsive Politics reporting team's whiteboard at NPR headquarters that was used to map out how Wellspring connects to other social welfare groups. (Click the enlarge button to see a full-size image.) John W. Poole/NPR hide caption

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The Au Sable River in Michigan is a popular place for fly fishermen and the heart of a debate unexpectedly influenced by largely invisible social welfare organizations. Christine Arrasmith/NPR hide caption

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The Olmsted Locks and Dam project is under construction on the Ohio River between Illinois and Kentucky. U.S. Army Corps of Engineers hide caption

itoggle caption U.S. Army Corps of Engineers

Tea Party activists gather on Capitol Hill in 2011. A surge in applications for 501(c)(4) status in recent years has revealed sometimes murky and contradictory rules governing the political activities of tax-exempt groups. Brendan Smialowski/AFP/Getty Images hide caption

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Republican U.S. Senate candidate Dan Winslow of Massachusetts wants same-sex married couples to have the same right to pool their money for political donations as other married couples. But the Federal Election Commission says the Defense of Marriage Act won't allow it. The constitutionality of DOMA is now before the Supreme Court. Charles Krupa/AP hide caption

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Senators Lisa Murkowski, an Alaska Republican, and Ron Wyden, an Oregon Democrat, discussing their new campaign finance legislation at an April 23, 2013 Capitol Hill news conference. Chip Somodevilla/Getty Images hide caption

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