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Citizens for Responsibility and Ethics in Washington, the campaign finance watchdog, is calling on the IRS to investigate Freedom Partners, a fund tied to the Koch Brothers that quietly doled out $235 million in grants to conservative groups last year.

As we reported in September, when news of the group's existence first broke, Freedom Partners made fairly novel use of a section of the tax code—501(c)(6)—reserved for trade associations, like the National Beer Wholesalers Association, or chambers of commerce. Freedom Partners calls itself a chamber of commerce, but it seems to be driven by political ideology rather a common business interest.

"I haven't seen a situation like this, and we watch that area pretty closely," Jim Clarke, the senior vice president of public policy at the Society of Association Executives, a sort of trade association of trade associations, told National Journal at the time. Most "dark money" political groups are organized as "social welfare" organizations under Section 501(c)(4) of the tax code, but 501(c)(6) status makes it easier for businesses to write off donations and may insulate groups from regulatory pressure.