Tom Steyer recently said in a video that "climate change is not just an important issue, it's the issue. And we need leaders who will take it seriously." Isaac Brekken/Getty hide caption

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Among outside groups — that is, not candidates or party committees — more than half of this cycle's political ads are financed by secret donors. iStockphoto hide caption

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SuperPAC NextGen Climate Action aired a 60-second ad about Iowa Republican Senate candidate Joni Ernst featuring two rich guys with a briefcase full of cash. YouTube hide caption

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People stand with signs on Capitol Hill as Senate and House Democrats called for an amendment to the Constitution aimed at curbing special interests' financial clout in elections. J. Scott Applewhite/AP hide caption

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The Ricketts family poses on the Chicago Cubs field in 2010, a year after they bought the team: Laura Ricketts (from left), Joe Ricketts, Marlene Ricketts, Todd Ricketts, Tom Ricketts and Pete Ricketts. Nam Y. Huh/AP hide caption

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Kent Sorenson says he was paid for his endorsement of Ron Paul in the 2012 presidential campaign — and that the exchange was hidden from the public. Charles Dharapak/AP hide caption

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Members of Congress are racing to take advantage of "franking" privileges, which allow them to replace postage with their signature. They are not allowed to use franking within 90 days of an election. iStockphoto hide caption

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