As third-party political advertising comes under increased scrutiny, Harvard Law School professor Elizabeth Warren and U.S. Senator Scott Brown signed an agreement Monday in an effort to limit the role such advertisements play in the upcoming Massachusetts Senate election.

The “Peoples’ Pledge” stipulates that if a third-party group runs an advertisement promoting a particular candidate, that candidate must donate 50 percent of the advertisement’s cost to a charity of the other candidate’s choosing.

The “Pledge” gives a candidate three days to make the donation. The campaigns will be responsible for monitoring advertisements and ensuring that penalties are paid.

“People have a right to make a decision in their Senate race based on what the candidates say and what the candidates do,” Warren said at a press conference Monday after the agreement was made. “Not based on what Karl Rove and the super PACs can accomplish.”

Brown expressed a similar sentiment in a statement released Monday.

“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 wrote.

The agreement—which pertains to all third-party television, radio, print, and online advertisements—follows two weeks of back and forth between the two frontrunners for the Massachusetts Senate seat.

On Friday, representatives from both campaign had been unable to settle on an agreement, with the Warren campaign arguing that the Brown camp’s proposal was not strict enough.

After tweaking the language of the proposal concerned with internet and “sham” advertising, the Warren campaign forwarded a new draft of the pledge to Brown Monday morning which he promptly signed.

The pledge is the first of its kind  since the Supreme Court ruled two years ago that the government cannot restrict independent organizations’ political spending. That ruling has given rise to large, well-backed political organizations called “super PACs” which have drawn much attention this election season.

Harvard professors and Massachusetts political analysts expressed skepticism about the enforcability of the “Peoples’ Pledge.”

Laws prohibit candidates from coordinating with third-party organizations, and questions remain over whether super PACs will have any incentive to stop advertising despite the fines candidates may face.

Institute of Politics Director C.M. “Trey” Grayson ’94 said that because the agreement has garnered much publicity, the effort may be effective in the short term. But Grayson said that he expects unaffiliated groups will ramp advertising as the election draws closer.

“They can look at the agreement, and understand what the candidates are doing,” Grayson said of third-party groups. “But do they really care? They aren’t on the ballot box.”

The candidates have sent co-signed letters to third-party groups asking them to cease all advertising on behalf of the candidates. Letters were also sent to local television and radio broadcasters requesting that they refuse to accept any advertisements from third-party groups pertaining to the race.

“You can ask them not to. You can say it’s going to hurt the people you’re trying to help,” Warren said. “Ultimately I don’t kid myself about this. The law is what the law is following Citizens United and these groups legally can come in and play their dirty tricks and try to do everything they can to shape this race.”

—Staff writer Nicholas P. Fandos can be reached at nicholasfandos@college.harvard.edu.