House Democrats are facing a backlash from some liberal and government reform advocacy groups over an exemption for the National Rifle Association (NRA) that was added to a campaign finance bill.

The U.S. Public Interest Research Group (PIRG) and the Sierra Club said Tuesday they will now oppose the Disclose Act, which Democrats are pushing as a response to the January Supreme Court decision that overturned limits on corporate and union contributions to political campaigns.

The opposition comes after Democrats agreed to a provision that would exempt the NRA and a few other large organizations from disclosure requirements that are central to the bill.

The exemption has rankled House liberals. Rep. Raúl Grijalva (D-Ariz.), co-chairman of the Congressional Progressive Caucus, said Tuesday he would try to mobilize members to oppose the NRA amendment and force it from the bill.

Democratic leaders hope to pass the campaign finance bill by the end of the week, but aides said they had yet to nail down the votes.

The decision by U.S. PIRG is significant because the group has been heavily involved in crafting the legislation, and it marks a split with several other good-government groups that support the legislation. PIRG research advocate Lisa Gilbert emphasized that the organization supports the goals of the legislation but that the carve-out for the NRA is too big a compromise.

"It's a sea change, and we hope that we'll be able to remove this exemption and support the bill," Gilbert said.

Rep. Chris Van Hollen (D-Md.), the chief sponsor of the legislation, defended the agreement.

"Our key objective in responding to the Supreme Court's radical ruling in Citizens United has been to increase transparency and disclosure, and the final bill achieves that goal," Van Hollen said in a statement.

"Reform in Washington is never easy - that is why powerful special interests are mobilizing against our effort to shine a light on campaign-related spending. The vast majority of Americans on the right, left and in the center support these efforts and I am confident that when the bill comes to the House floor it will pass."

Leadership aides pointed to five good-government groups that have endorsed the revised legislation, including the NRA exemption. Common Cause, Public Citizen, Democracy 21, the League of Women Voters and the Campaign Legal Center are all supporting the bill.

It remained unclear Tuesday whether House liberals would ultimately withhold their support for the bill - which most broadly support - over the NRA exemption. The Progressive Caucus sent an e-mail to gauge member support for taking a stand against the amendment. Leadership aides were also concerned about labor unions, which have yet to endorse the bill and were said to be upset at the exemption for the NRA.

Grijalva and Rep. Carolyn McCarthy (D-N.Y.), a leading supporter of gun control, both said they would vote against a manager's amendment including the NRA exemption but had yet to decide on the final bill.

"Even final passage is jeopardized for me," Grijalva said. He said the overall bill "continues to be a good piece of legislation" but that the "bad taste" of the exemption had caused consternation among members.

"We don't think it's equitable," he said.

McCarthy said she was less concerned about the exemption for the NRA than about the perception that the gun lobby had such a heavy influence on the legislation. The NRA, she said, already discloses its involvement in political campaigns, unlike many large corporations that try to disguise their influence through political action committees. When the NRA sends out mailers and runs ads, McCarthy said, "you know where that information is coming from."

The NRA on Tuesday confirmed it would stand down from lobbying against the bill because of the exemption. In a statement, the group said that as long as the provision remains intact, the NRA "will not be involved in final consideration of the House bill."

The Sierra Club, a leading environmental advocacy group, had not taken a position on the Disclose Act before the NRA provision was added. A spokesman, David Willett, said the group had concerns both with that provision and others he said might limit the Club's non-electoral grassroots activities.

"We are supportive of the underlying goals," he said.

The Brady Campaign to Prevent Gun Violence, the U.S. Chamber of Commerce and the National Right to Life Committee have also come out against the bill.

The NRA carve-out, negotiated by gun-rights supporters in Congress, would exempt organizations that have more than 1 million members, have existed for more than 10 years and raise a maximum of 15 percent of contributions from corporations. Democratic aides said the AARP and the Humane Society were the only other organizations believed to qualify for the exemption.

The president of the Brady Campaign, Paul Helmke, said lawmakers should be more concerned about limiting the influence of smaller interest groups than giants like the NRA.

"This is such a completely backward type of proposal," he said. "It makes no sense to exempt larger organizations who spend more."

The legislation has two Republican sponsors: Reps. Mike Castle (Del.) and Walter Jones (N.C.). Most other GOP lawmakers are expected to oppose it.

Senate Republican Leader Mitch McConnell (Ky.), a champion of gun rights, lambasted the NRA exemption as a "backroom deal" reminiscent of the healthcare debate.

"Taxpayers are still fuming over a healthcare process where their money was thrown around like a high roller in a hotel lobby to win last-minute votes, and now the same backroom dealing is being repeated with their freedom of speech," McConnell said in a statement. "Just as it wasn't the Democrats' money to offer in the healthcare debate, free speech isn't theirs to ration out to those willing to play ball - it's a right guaranteed by our First Amendment to all Americans."