Sen. Jon Tester joined Sen. Ron Wyden of Oregon on Monday in launching an effort to force a Senate vote on a recent federal decision enabling certain tax-exempt groups to avoid reporting major donors to the IRS.

Tester and Wyden, D-Oregon, contend that undoing the U.S. Treasury Department’s recent decision is needed to prevent large donors from hiding financial contributions to special interests.

“We must crack down on the dark money flooding our political system, that’s why we are taking an aggressive approach and introducing this legislation to protect our democracy and hold special interests accountable with transparency,” Tester said in a statement. “We cannot allow special interests to hide in the darkness.”

In July, the Treasury Department announced that it would no longer require certain tax-exempt groups to list their financial donors to the IRS. The move was praised by conservatives and blasted by those fearful of dark money’s sway over American politics.

The change protects the privacy of donors who make dark money contributions of more than $5,000 to politically active, tax-exempt organizations, including Planned Parenthood and the National Rifle Association.

While supporters see the move as a win for free speech, critics say the rule will make it easier for politically active lobbies to conceal large contributions, including those from foreign countries.

“This measure reverses course on an undemocratic Trump agenda to empower shadowy groups that seek to buy our elections,” Wyden said in a statement. “It couldn’t come at a more important time.”

Wyden noted that over the next few weeks, political ads will flood the airwaves. The new tax-exempt rule will further conceal who paid for those ads, be it a foreign government or a wealth donor.

“Opposition against my resolution with Sen. Tester is an endorsement of the Trump administration’s plan to further cripple campaign finance transparency,” Wyden said.

Tester and Wyden introduced what’s known as a Congressional Review Act, which requires members of Congress to introduce a resolution of disapproval within 60 days from when the final rule was issued.

After 20 calendar days, the resolution can be discharged from the Senate Finance Committee without a committee vote. At that point, a motion to proceed to the resolution may be made on the Senate floor if it’s supported in writing by 30 Senators.

Tiffany Muller, president of the End Citizens United Action Fund, praised the resolution.

“Voters deserve to know who’s behind the seemingly unlimited money that’s trying to influence their vote and their government,” Muller said. “It would shine a much-needed light on political nonprofits that spend hundreds of millions of dollars every election cycle in the shadows.”