Senate Majority Whip Dick Durbin (D-Ill.) called on the IRS on Tuesday to quickly investigate the tax status of Crossroads GPS and other organizations that are spending tens of millions of dollars on campaign ads without disclosing the sources of the money.

Crossroads GPS is one of several nonprofit groups set up this year under a section of the tax code that is reserved for nonprofits whose primary purpose is not political. The groups have primarily benefited Republicans.

“I write to urge the Internal Revenue Service to examine the purpose and primary activities of several 501 (c)(4) organizations that appear to be in violation of the law,” Durbin wrote in a letter to IRS Commissioner Douglas Shulman. Crossroads “has spent nearly $20 million on television advertising specific to Senate campaigns this year,” he added. But unlike 527s, the donors to 501(c)(4) groups can be kept secret.

Several watchdog groups and other Senators, including Finance Chairman Max Baucus, had previously asked the IRS to look into spending by these groups this election cycle.