Video: Leah McGrath Goodman on the Dark Money Flooding U.S. Elections

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As a result of the U.S. Supreme Court's Citizens United v. Federal Election Commission judgment in 2010, individuals—and big corporations—received a carte blanche to make unlimited anonymous financial donations to "nondisclosing" organizations, increasingly nonprofit groups whose primary mission is defined as "social welfare."

This year, the greatest amount of dark money, more than $50 million, has been spent on boosting Republicans and defeating Democrats. Outside of the realm of dark money, it's the reverse: The majority of funds—more than $100 million—have been spent on defeating Republicans, with pro-Democrat TV ads dominating top Senate races in the first two weeks of September.