WASHINGTON (Reuters) - A Democratic congressman filed a lawsuit on Thursday against the Federal Election Commission, widening efforts to expose anonymous campaign donations as the 2012 U.S. election gets under way.

Representative Chris van Hollen, the senior Democrat on the House of Representatives' Budget Committee, filed the suit challenging FEC regulations which he said undermined the degree of campaign finance disclosure demanded by U.S. law.

"The absence of transparency will enable special interest groups to bankroll campaign initiatives while operating under a veil of anonymity," Van Hollen said in a statement.

Democrats claim that attack ads worth millions of dollars were unleashed against them in the 2010 congressional election, after a Supreme Court ruling that allowed unlimited spending by corporations, labor unions and other groups.

Democratic President Barack Obama was outspoken in his criticism of the ruling and the White House said an executive order had been drafted to force federal contractors to disclose anonymous campaign donations.

Obama, who officially announced his 2012 reelection campaign this month, has not yet signed the order, which would also identify donations to "third party entities" that Democrats blame for some of the most aggressive ads.

Republicans criticized the move as an attempt to muzzle critics of Obama's administration.

Van Hollen is pressing the FEC to alter regulations installed after last year's "Citizens United" Supreme Court ruling to force them to respect a bipartisan 2001 campaign finance law that he says would bar anonymous donations.

"If this standard had been adhered to, much of the more than $135 million in secret contributions that funded expenditures in the 2010 congressional races would have been disclosed to the public," he said. The figure was based on estimates by the Center for Responsive Politics, an independent research group in Washington tracking money in U.S. politics.

Republicans won control of the House of Representatives in the election last November.

An FEC spokeswoman declined to comment on the lawsuit, which was filed in the Washington D.C. District Court.

A Democratic congressional source said that after failing to push back against the "Citizens United" decision through legislation last year, Democrats were now using other routes to attempt to restore campaign finance transparency.

But the source said the appearance of coordinated action was a coincidence.

Obama raised $750 million in his victorious 2008 election campaign and the 2012 election is expected to be the most expensive in U.S. history.

Obama's fund-raising machine was helped significantly by small donations from grassroots supporters, helping offset a traditional financial edge enjoyed by Republicans because of their closer ties to the business community.

(Editing by Christopher Wilson)

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