Democratic members of the Senate Finance Committee want the Internal Revenue Service to investigate whether nonprofit advocacy groups are violating rules limiting their political spending, The New York Times and The Wall Street Journal write.

Advocacy groups categorized under Section 501(c)(4) of the tax code have become increasingly popular vehicles for campaign spending, particularly on behalf of Republicans. Such organizations are not required to identify their donors as long as their purpose is not primarily political.

In a letter sent to the IRS commissioner on Tuesday, Montana Sen. Max Baucus, chairman of the Finance Committee, asked the agency to assess whether advocacy groups are using the tax code “to eliminate transparency in the funding of our elections.”

A spokesman for American Crossroads and Crossroads GPS, two groups that have been involved in efforts to defeat Democratic candidates this fall, say they abide by the tax code and their missions are focused on promoting specific issues rather than being explicitly partisan.

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