WASHINGTON — An expansive onslaught of negative political advertisements in congressional races has left many incumbents, including some Republicans long opposed to restrictions on campaign spending, concluding that legislative measures may be in order to curtail the power of the outside groups behind most of the attacks.

While Democrats have long denounced a 2010 Supreme Court decision that opened the gates on unlimited spending on advertisements, some Republicans are now growing more disenchanted with the system that allowed the barrage of ads, often by shadowy groups, and the effects it has had on what they see as a sullen electorate.

"Once we get back, those that do get re-elected will all be commiserating about all the negative ads," said Rep. Joe Heck, R-Nev., who faced ads accusing him of voting against a rape-crisis center and against money to help victims of domestic violence, among other things. "And that will start the groundswell for reform."

Rep. Dan Lungren, R-Calif., the chairman of the House Administration Committee, which has jurisdiction over campaign-finance issues, has also been a target of negative advertisements. He has drafted legislation that he said would force more responsibility for the tone and messages of the campaign onto the candidates and political parties and away from the third-party groups. The staff of Sen. Lisa Murkowski, R-Alaska, is also working on proposals.

The 2010 Supreme Court ruling, Citizens United v. Federal Election Commission, cleared the way for unlimited campaign donations to a new breed of "super PACs" from corporations, unions and wealthy contributors. It was expected to give an unalloyed advantage to Republicans, who have a deeper bench of rich individuals and corporations willing to finance candidates.

The decision has appeared to be benefiting Republicans overall this election cycle, as Republican money has poured into the presidential contest. Democrats say their third-party allies have also been outspent, by about two to one, in Senate campaigns. But the impact of Citizens United has come with complications, with some Republican incumbents in the House at a disadvantage.

Earlier this month, before Republicans surged ahead with an additional $25 million, the total spending and reservations for ad time in the House campaigns had been dead-even at $89 million, according to the National Republican Congressional Committee. Conservative donors were confident that the House Republican majority was secure and sent their money elsewhere. Democratic donors, including unions and environmental groups such as the League of Conservation Voters, have been more strategic, concentrating their fire on a handful of vulnerable House Republicans.

Lungren said the attacks on him began just months after the 2010 election, with radio advertisements and automated phone calls. They have accelerated into an onslaught of television commercials in what has become the most expensive House race in the country. Lungren's opponent is Ami Bera, a doctor and Democrat.

Lungren said the 2012 experience could be transformative for other Republicans who have spent the past six months enduring the grim piano music and disconsolate faces of "voters" in negative ad after ad, sometimes against them, sometimes on their behalf but always without their sign-off.

"We had to see how this worked out for a cycle," he said.

Sen. John Cornyn, R-Texas, who runs the group charged with electing Republicans to the Senate, has said he thinks it would be worthwhile to examine the campaign-finance system after the election.

"Revisiting the federal fundraising restrictions and coordinated limits on both parties, and even smaller, common-sense steps like requiring electronic filing for federal candidates, are a few good things that could be looked at next year," he said in an email.

But Sen. Mitch McConnell of Kentucky, the Republican leader, has opposed efforts to require the unveiling of secret donors.

"The courts have said that Congress doesn't have the authority to muzzle political speech," McConnell said in a talk in June to the American Enterprise Institute on campaign-finance restrictions.

Since the Supreme Court ruling, Democrats in Congress have tried to legislate curbs on campaign advertising. Rep. Chris Van Hollen, of Maryland, and Sen. Charles Schumer, of New York, drafted the Disclose Act, which passed the House narrowly in 2010 and received majority support in the Senate but fell one vote short of the 60 needed to break a Republican filibuster.

But the barrage of advertisements in the 2012 campaign has the potential to change the political dynamic. Since the beginning of the year, more than 3 million political advertisements have been broadcast, according to Kantar Media, a research firm that tracks political advertising, and the final weeks of the campaign are likely to be the most intense.

As of Thursday, 78,782 ads had run in Las Vegas, a record for any media market. In all of 2008, the Las Vegas media market broadcast 29,241 ads.

Cleveland, host to a pivotal contest in the presidential race, a well-funded Senate campaign and the second-most expensive House race in the country, came in second with 70,520 spots, up from 30,307 in 2008.

Billings, Mont., population 105,000, has been treated to 31,241 ads, triple the number four years ago. North Dakota's two tiny media markets have broadcast 37,783 ads chasing about 320,000 voters.

Heck, of Nevada, who said he had no idea who was behind some of the ads against him, said he was ready to back legislation at least mandating the timely disclosure of contributors to the third-party groups saturating the airwaves. Many Republicans have suggested exchanging greater transparency about the ads for a lift on the legal spending cap on individual donors, something that Democrats would likely balk at.

"I do think that sunlight on this is the antidote," said Rep. Steve King, R-Iowa, a staunch defender of the Supreme Court decision, who was hammered this year by ads like nothing he had experienced in previous elections. "So that sounds like something we ought to look at. When you cap contributions, you're also limiting freedom of speech. So when those donors max out, they put resources into super PACs, which are less accountable to candidates."

Lungren's bill would remove all limits on individual contributions to candidates and all limits on transfers between candidates and the political parties, but would mandate the immediate disclosure of contributions.

Rep. Robert Dold, a moderate Republican who represents the 10th District in Illinois and is fighting for his political life, said: "I think what we're going to find as history takes a look is that the Citizens United case diluted the voice of the average voter with the amount of advertising from outside groups. There are going to be those that say that was a good thing, but I do think the people of the 10th District deserved better."

Information from Seattle Times archives is included.