Looking to rein in the independent organizations that have dumped millions of dollars into the already costly Massachusetts Senate contest, Sen. Scott Brown (R) and his Democratic challenger Elizabeth Warren announced an agreement yesterday aimed at limiting advertising in the race to their respective campaigns.

The agreement, which appears to be the first of its kind, seeks to limit advertising by so-called SuperPACs and 501(c) nonprofit groups that has proliferated since the 2010 Supreme Court Citizens United ruling -- as well as advertising from 527 groups and national and state political parties.

Brown has already felt the lash of environmental groups, which have spent heavily on ads attacking his votes.

According to the document, which the candidates have named the "People's Pledge," if an outside organization violates the ban and airs an ad -- be it on broadcast television, cable, radio or the Internet in support or opposition to either candidate -- Warren or Brown will pay 50 percent of the cost of the ad to a charity of their opponent's choice.

The Warren campaign also provided letters she and Brown jointly sent to radio and TV station managers asking them to refuse to air such ads and to outside groups requesting that they comply with the agreement.

"In short, your spending will damage the candidate you intend to help," the letter to third-party groups states.

Both candidates sought credit for the ceasefire, which Brown had initially proposed. Warren's campaign took credit for pursuing a formalized agreement.

"This is a great victory for the people of Massachusetts, and a bold statement that puts Super PACs and other third parties on notice that their interference in this race will not be tolerated," Brown said in a statement. "This historic agreement means the candidates will be in control of their own campaigns and accountable for what is said. My hope is that the extreme liberal groups who planned to pollute the airwaves with their false and misleading ads in support of Professor Warren can now pack their bags and find someplace else to do their dirty work."

In a separate statement, Warren said: "With our joint agreement we have now moved beyond talk to real action to stop advertising from third party groups. But both campaigns will need to remain vigilant to ensure that outside groups do not try to circumvent what is an historic agreement. This can give Massachusetts voters a clear choice come Election Day."

Whether those organizations -- which have already poured millions of dollars of advertising into the race -- comply with the Senate candidates' agreement remains to be seen.

Attorney Brett Kappel, who focuses on campaign finance law at the Washington, D.C.-based firm Arent Fox, said the pledge is "not realistic at all."

"All they can do is agree to ask their supporters not to run ads or contribute to groups that do," Kappel said. "Unfortunately, these groups don't necessarily care what either Senator Brown or Professor Warren think -- they work for people who are more concerned with which party controls the Senate after the 2012 election, not which candidate wins a particular race. They'll keep running ads until they are no longer effective or become counterproductive."

In fact, Steven Law, president of the conservative group American Crossroads, which is affiliated with the Super PAC Crossroads GPS, criticized the agreement for failing to include potential third-party outlets, like telephone banks.

"Because the agreement allows union phone banks, direct mail, and get-out-the-vote drives -- all union core specialties -- Warren's latest agreement has loopholes the Teamsters could drive a truck though, the longshoremen could steer a ship through, the machinists could fly a plane through, and government unions could drive forklifts of paperwork through," Law said in a statement.

But the League of Conservation Voters, whose LCV Action Fund has aired several ads critical of Brown's stances on environmental legislation, said it plans to abide by the agreement.

"Since the disastrous Citizens United decision two years ago, there has been an unprecedented flood of corporate special interest money into the political process, much of it from oil companies and other polluters," LCV Senior Vice President of Campaigns Navin Nayak said in a statement. "While we cannot take directions from any candidate on our independent activities, we are inclined to respect the People's Pledge agreed to by Elizabeth Warren and Scott Brown and we hope that Scott Brown will honor his end of the deal when Crossroads and the Koch Brothers inevitably break it."

Lisa Gilbert, who is deputy director of Public Citizen's Congress Watch, noted the candidates' agreement -- while potentially sidelining millions of dollars in attack ads -- also provides another benefit, essentially giving back full control of the messaging process to the campaigns themselves.

"If all those players just get out, candidates have the ability to handle their own campaign," Gilbert said, suggesting if the pledge succeeds in pushing out third-party advertising, it could catch on more widely in the 2014 cycle.

"I think a lot of people are still seeing how the lack of new rules [under Citizens United] and all these new entities play out, but if it works for Scott Brown or Elizabeth Warren ... I think people will be jumping all over it," she said.

Even without third-party spending, the Massachusetts Senate race promises to be one of the most expensive -- and one of the most competitive -- of the 2012 cycle. The most recent public poll, taken in early December, showed Warren with a 7-point lead over the incumbent.

The Federal Election Commission has yet to post year-end fundraising reports for either Warren or Brown -- they are not due until next week -- but the Boston Globe reported earlier this month that Brown claimed to have raised a total of $8.5 million for 2011, with $12.8 million on hand. The newspaper reported that Warren, a Harvard law school professor and the architect of the Consumer Financial Protection Bureau, raised $8.8 million since entering the race last fall, including $5.7 million in the fourth quarter. Warren's campaign told the Globe it had $6 million on hand.

Brown spent $15.5 million to win the January 2010 special election to replace the late Sen. Edward Kennedy (D). His Democratic opponent, state Attorney General Martha Coakley, spent $9.6 million.

Public Citizen and other government reform groups unveiled a new effort last week aimed at reversing the effects of the 2010 Supreme Court decision in Citizens United ruling, coinciding with the decision's second anniversary. Public Citizen, as part of the Corporate Reform Coalition is pushing for the Securities and Exchange Commission to set rules on corporate political spending, including the disclosure of expenditures.

People for the American Way is sponsoring a forum today in Washington, D.C., to discuss options for a constitutional amendment to overturn the court's decision. The panel, which includes Sen. Tom Udall (D-N.M.) and Reps. Ted Deutch (D-Fla.) and Keith Ellison (D-Minn.), begins at 10 a.m. in Room 106 of the Senate Dirksen Office Building. Click here to watch.