Rep. Chris Van Hollen (D-Md.) filed a challenge to campaign spending rules in federal court Thursday, charging that the regulations do not adequately implement the portions of the 2002 McCain-Feingold law requiring disclosure of political donations.

The lawsuit challenges regulations at the Federal Election Commission that allow groups organized as nonprofits to spend money on candidate ads ahead of an election without revealing their financial backers.

“The disclosure of campaign-donor information is essential to our democracy,” Van Hollen said in a statement announcing the lawsuit. “The absence of transparency will enable special interest groups to bankroll campaign initiatives while operating under a veil of anonymity.”

Supporters of the lawsuit say interest groups ran $135 million worth of campaign ads in the last election cycle without revealing the source of the money.

The White House confirmed Wednesday that it is drafting an executive order that would require companies seeking federal contracts to disclose their contributions to politicians and advocacy groups involved in elections.

Taken together, the moves signal that Democrats are widening their effort to challenge rules for election spending in the wake of the last year’s landmark Supreme Court decision that struck down prohibitions on independent campaign spending by corporations, unions and nonprofit groups. The court’s decision in Citizens United v. Federal Election Commission allowed many interest groups that are not required to reveal donors to spend money on campaign ads.

Republicans attacked both measures as politically motivated. Rep. Sam Graves (R-Mo.), chairman of the House Small Business Committee, sent a letter to President Obama on Thursday saying the proposed executive order would “politicize public contracting and infringe on the free speech rights of American citizens.”

Bradley Smith, a founder of the Center for Competitive Politics, which advocates for fewer restrictions on campaign spending, said: “The Democrats at every level are trying to do everything they can to limit the speech of their political opponents. None of it has anything to do with corruption at all.”

During the midterm election last year, Obama and other Democrats spoke often about the need for public scrutiny of donors to interest groups, most of which were targeting Democratic candidates.

Democrats tried to pass a bill, dubbed the “Disclose Act,” to force interest groups to reveal donor identities, but the measure failed to get the required 60 votes to pass a procedural test in the Senate. The executive order and the Van Hollen lawsuit, if successful, would implement some of the provisions of that bill without requiring the approval of Congress, which is unlikely to act now that the House is under Republican control.

Van Hollen’s legal team includes the advocacy groups Democracy 21 and the Campaign Legal Center, which also represented former congressman Chris Shays (R-Conn.) in similar suits over the FEC’s implementation of the McCain-Feingold law. Those lawsuits forced the agency to rewrite 19 regulations.

The groups also filed a petition with the FEC to rewrite other regulations that were not challenged in the lawsuit because the statute of limitations had passed. Its chances of success are slim given a spate of partisan deadlocks on the commission over disclosure rules.