WASHINGTON (Reuters) - Forty-five public interest groups expressed opposition on Wednesday to a Democratic bill requiring greater disclosure in campaign financing, saying an exemption for the powerful National Rifle Association was unjust.

“There is no legitimate justification for privileging the speech of one entity over another, or of reducing the burdens of compliance for the biggest organization yet retaining them for the smallest,” they wrote Speaker of the House of Representatives Nancy Pelosi.

The largely liberal signers of the letter included the Alliance for Justice, League of Conservation Voters and Citizens for Responsibility and Ethics in Washington.

Their opposition raised a new hurdle for the White House-backed measure to blunt the impact of a U.S. Supreme Court ruling this year that opened the gates for unlimited campaign spending by corporations, unions and other groups.

The bill, which backers have hoped would take effect before the November congressional elections, would require unprecedented disclosure of money in politics.

Supporters had hoped to win House passage as early as this week, but it is now unclear if they have the necessary support in the Democratic-led chamber. Representative Chris Van Hollen, head of the House Democratic campaign committee, is chief sponsor.

“At this point, I’m assuming that Van Hollen feels he doesn’t have the votes yet,” said Craig Holman of Public Citizens’ Congress Watch, a backer of the bill. “But I still feel that he will get there because this is so important.”

“The legislation still provides what the public desperately needs ... full disclosure of corporate and wealthy funding sources behind express advocacy ads,” Holman wrote in an op-ed in the National Journal on Wednesday.

Several other public interest groups also back the bill, “The Disclose Act,” saying the exemption was limited and would help get pro-NRA lawmakers to vote for it.

Doug Thornell, a Van Hollen spokesman, said, “The DISCLOSE Act will increase transparency and disclosure, and ensure the American people know who is spending money on elections.”

FACES TOUGH FIGHT IN SENATE

Even if the bill wins House approval, it faces a tough fight in the Democratic-led Senate where Republicans are virtually certain to try to raise a procedural roadblock.

In order to muster support for the bill and end NRA opposition to it, top House Democrats agreed this week to grant the 4-million member organization and a few other large groups a partial exemption.

Under the accord, the NRA, one of the most influential lobbying groups on Capitol Hill, would not have to disclose its top donors on its campaign ads. The NRA, however, like other groups, would have to put its own name on any of its spots.

The proposed exemption on donor disclosure would apply only to groups that have at least 1 million members, are at least 10 years old and receive less 15 percent or less of its funds from corporations or unions.

In addition to the NRA, the only other groups that would apparently qualify for the exemption are the AARP, a top advocacy group for the elderly, and the Humane Society.