David Koch speaks at the Defending the American Dream summit in 2015. He and his brother Charles lead a conservative political network that plans to spend hundreds of millions of dollars on the 2016 elections. Paul Vernon/AP hide caption

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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

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Sen. Rand Paul, R-Ky., center, meets with members of the Londonderry Fish and Game Club in Litchfield, N.H., on Jan. 14. Paul was one of three GOP presidential hopefuls who attended Sunday's semiannual gathering of David and Charles Koch's donor network in California. Jim Cole/AP 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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David Koch, executive vice president of Koch Industries, attends an event at The Economic Club of New York last year. Mark Lennihan/AP hide caption

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