WASHINGTON — Congressional Democrats are pushing hard for legislation to rein in the power of special interests by requiring more disclosure of their roles in paying for campaign advertising — but as they struggle to find the votes they need to pass it they are carving out loopholes for, yes, special interests.

In a deal that left even architects of the legislation squirming with unease, authors of a bill intended to counter a Supreme Court ruling allowing corporations and unions to pour money directly into campaign commercials carved out an exception this week for the National Rifle Association, one of the most powerful lobbying groups in Washington.

The resulting uproar over special treatment for the pro-gun group led Democrats on Thursday to expand the exception to cover even more interest groups as they tried to secure votes for the measure, which is opposed by most Republicans. But with other powerful groups also weighing in and no assurance that Democrats had the votes they need, House leaders decided late Thursday to put off a planned Friday vote on the campaign bill, increasing doubts about whether Congress can enact it in time for this year’s elections.

The episode has highlighted again the difficulty President Barack Obama has had in fulfilling his promise to diminish the power of lobbyists and interest groups, and how deeply entrenched the groups are in the process of shaping policy proposals into detailed legislation. Obama has sought to make the effort to fight back against the Supreme Court ruling — which could allow corporations, industry groups and unions greater opportunities to sway voters — a key part of his party’s platform for this year’s midterm elections.

Backers said the NRA deal was the only way to win enough support among House Democrats, many of whom refuse to take any action that could rile the gun lobby, and impose regulations they say are necessary to prevent corporations and other organizations from hiding behind front groups and flooding the airwaves in this year’s elections.

"It is not the bill I would have written, but it is better than nothing," said Rep. Jim McGovern, D-Mass. "If we are going to be Swift-boated, we should be able to know who is doing it."

Even as authors watered down the measure, moderate Democrats expressed concern about crossing another high-powered advocate, the U.S. Chamber of Commerce, which is strongly opposed to the bill and is threatening to rate lawmakers on what position they take on it. At the same time, members of the Congressional Black Caucus were questioning how the measure might fall on groups like the NAACP.

Rep. Chris Van Hollen of Maryland, head of the House Democratic campaign arm, developed the measure in response to the court ruling this year in Citizens United v. the Federal Election Commission, which found that the government may not ban political spending by corporations in candidate elections. The proposed legislation would subject corporations, unions and other nonprofit groups that engage in political activity to significant new disclosure rules.

The leaders of entities covered by the law, including the chief executives of corporations that engage in campaign advertising, would have to appear at the end of the advertisements and make the now familiar statement that they approve the message. In addition, when the advertisements come from advocacy groups, the top five contributors to the cost of the ads covered under the new rules would have to be listed.

Even if the House can approve the measure, it faces an uncertain future in the Senate, where the agenda is crowded and the Republican leader, Sen. Mitch McConnell of Kentucky, is a fervent opponent of campaign finance restrictions. But top Democrats have assured their House counterparts that they could pass a bill, which if signed into law would most likely face immediate constitutional challenges.

"I think that the way that they’ve carved out some people and left others under the law again clearly violates the Constitution and violates what the court made clear that Congress should not do," Rep. John A. Boehner of Ohio, the Republican leader, said Thursday.

Democrats say the proposal does not infringe on speech but simply allows the public a better idea of who is financing the advertising campaigns, a principle they say that Republicans formerly supported.

"I don’t know why, if they’re so proud of their point of view, they don’t want to stand by their ads," Speaker Nancy Pelosi said Thursday about groups objecting to the measure.

But moving ahead with the measure has proved vexing, and the decision to provide an exemption for the NRA to win the votes of one group of Democrats was particularly hard for many other lawmakers to accept, making it difficult for House leaders to secure the required votes.

"The remedy does not treat the sickness; it only creates another ill," Rep. Mike Quigley, D-Ill., said.

Under the initial compromise, a provision was added to the measure that, without naming the NRA, was clearly tailored to fit it and other advocacy goliaths like AARP. The provision was to apply to select nonprofit groups that have existed for more than 10 years, have more than 1 million members with some in each state and get no more than 15 percent of their financing from corporations or unions.

Critics said the authors were picking winners and losers.

"Guns for all, free speech for some," said R. Bruce Josten, a top Chamber of Commerce official who called the measure blatantly political and an effort to protect Democratic incumbents.

Responding to the outcry, the NRA compromise was changed Thursday to exempt groups with 500,000 or more members, further diminishing the reach of the legislation even as it appealed to supporters of additional advocacy groups.

Still, to many liberals, the NRA deal rankled.

"I just thought we were making the NRA too powerful," said Rep. Raul M. Grijalva, D-Ariz., a leader of the House Progressive Caucus, of the initial deal. "I understand if we don’t do anything we are going to get drowned with corporate money in this election, but at the same time, it kind of gnaws at you."

Rep. Michael N. Castle of Delaware, one of two House Republicans backing the campaign act, said partisanship had overtaken what he initially considered a fairly straightforward measure.

"To me, it is a relatively simple request to make corporations, unions and nonprofits go through the same steps that political parties go through," he said. "And yet it is running into all kinds of opposition."