Washington, D.C., Mayor Muriel Bowser last month. Larry French/Getty Images hide caption

Democratic mayoral candidate Jim Kenney benefited from a super PAC that helped him win in the city's Democratic primary election. Matt Slocum/AP hide caption

Bernie Sanders' and Hillary Clinton's fundraising numbers got a lot closer last quarter, thanks in part to Sanders' huge small-donor base. Joe Raedle/Getty Images hide caption

A volunteer with the Koch-funded Libre Initiative directs people in 2014 as groceries are distributed at a food bank it partially sponsors in San Antonio. Eric Gay/AP hide caption

Pope Francis reads from his prepared remarks as he addresses a joint meeting of Congress. Evan Vucci/AP hide caption

Supporters of Democratic presidential candidate Sen. Bernie Sanders, I-Vt., cheer the candidate at the New Hampshire Democratic convention Saturday. Jim Cole/AP hide caption

Rick Perry ended his presidential campaign Sept. 11, but there was $13 million left in the bank of a superPAC supporting him. The superPAC says it's given donors their money back. Scott Olson/Getty Images hide caption

Former House Majority Leader Rep. Tom DeLay, R-Texas, in 2011. Ben Sklar/Getty Images hide caption

Former Massachusetts Gov. Mitt Romney in California last week. Justin Sullivan/Getty Images hide caption

Some superPAC donors are hiding from public scrutiny. iStockphoto.com hide caption