WASHINGTON — An 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 has allowed the barrage of ads, often by shadowy groups, and the effects it has had on what they see as a sullen and disenchanted electorate.

“Once we get back, those that do get re-elected will all be commiserating about all the negative ads,” said Representative Joe Heck of Nevada, a Republican 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.”

Representative Dan Lungren, a California Republican and the chairman the House Administration Committee, which has jurisdiction over campaign finance issues, has 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 the political parties and away from the third-party groups. The staff of Senator Lisa Murkowski, Republican of Alaska, is also working on proposals.