Republicans want the IRS to revoke the Natural Resources Defense Council's tax-exempt status because of an ad campaign it is running against Sen. Mark Kirk and other GOP senators up for re-election in 2016.

The NRSC, formerly the National Republican Senatorial Committee, said the environmental group's million-dollar ad campaign came out of its 501(c)(3) outfit rather than its 501(c)(4) advocacy arm, the Natural Resources Defense Council Action Fund. The NRSC said using the former structure violated federal laws that prevent charities from undertaking political activities, while allowing donors to deduct contributions in annual tax filings.

"The question is, why would the 501(c)(3) pay for the ad? The only conceivable explanation is that [Natural Resources Defense Council] consciously is enabling its donors to take tax deductions in exchange for political contributions in violation of federal law," Matthew Raymer, general counsel for the NRSC, wrote last week in the 11-page complaint to the IRS.

The 30-second ad slams Kirk of Illinois, who faces a tough road for re-election in the traditionally Democrat-voting Prairie State, for a June vote on a bill that would block spending to implement and enforce an Environmental Protection Agency rule limiting carbon emissions from power plants that is due to be finalized this summer. The ad flashes Kirk's office phone number and implores viewers to "tell Sen. Kirk to stop protecting polluters and start protecting our kids."

David Goldston, director of government affairs with the Natural Resources Defense Council, said Monday the "complaint is an effort to distract attention from Senator Kirk's vote."

"Senator Kirk has yet to explain why he cast a vote that would prevent cleaning up power plant pollution — pollution that can lead to asthma attacks, as well as pollution that worsens climate change. This public education ad was designed to inform Sen. Kirk's constituents about a critical vote he cast that deals with an issue of utmost concern to all Illinoisans. This is one of the essential roles for advocacy groups. The complaint is without merit," Goldston said in an emailed statement.

In the complaint, the NRSC also noted the environmental group has run ads blasting GOP Sens. Rob Portman, of Ohio, and Pat Toomey, of Pennsylvania, as well as Republican state senators in North Carolina. It said that, with the campaign against Kirk, the ads amounted to more than $2 million in spending over the last 18 months.

"This ad, the latest in a string of multi-million dollar political activities, leaves no doubt that NRDC has forfeited its 501(c)(3) status," Raymer said.