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

itoggle caption Mark Lennihan/AP