Updated: 6:31 p.m.

Sen. Scott Brown (R-Mass.) and Harvard Law School professor Elizabeth Warren agreed today to attempt to limit third parties from advertising in the Massachusetts Senate race.

Warren, the presumptive Democratic nominee, and Brown signed a pledge to pay a penalty if any third-party group advertised on their behalf or against their opponent. The agreement says that the candidate who benefits from the third-party advertising would pay 50 percent of the cost of the advertising buy to a charity of his or her opponent’s choice.

The agreement attempts to limit the advertising activity of corporations, super PACs, 527s and the state and national party committees.

Early indications were that national parties were willing to play ball.

“Provided Democrats honor this agreement, our side intends to, as well,” one national Republican official said.

A national Democratic source indicated they would do the same.

The state Republican and Democratic parties intend to comply with the agreement as well, officials from both camps told Roll Call tonight.

A spokesman for Republican-aligned group American Crossroads didn’t respond directly to questions about whether the group’s sister organization, Crossroads GPS, would continue to buy ads in the Massachusetts Senate race. Crossroads GPS has spent about $1.1 million on anti-Warren issue ads in the state.

But American Crossroads President Steven Law questioned the pact.

“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 through,” Law said in a statement.

The League of Conservation Voters, which spent almost $2 million on an anti-Brown ad, released a statement today that sounded like it was also willing to get onboard with the agreement.

“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,” LCV Senior Vice President of Campaigns Navin Nayak said in a statement.

Warren and Brown, who have been locked in a public-relations battle over the issue in recent weeks, hailed the agreement.

“With our joint agreement we have now moved beyond talk to real action to stop advertising from third party groups,” Warren said in a statement. “But both campaigns will need to remain vigilant to ensure that outside groups do not try to circumvent what is an historic 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.

It’s unclear who will benefit more from the agreement over the long term. Because both candidates have substantial war chests and the ability to raise a lot of money quickly, losing help from outside groups won’t hamper Brown or Warren’s ability to get a message out to voters.

Still, if the agreement holds, it represents a groundbreaking bipartisan move against the flood of outside money that has entered the political process in recent years.

“It’s certainly innovative. I can think of no precedent for this type of agreement. The question is whether it is truly enforceable,” said Anthony Corrado, a professor at Colby College and an expert in campaign finance.

“Legally, there is no way they can limit these groups from participating,” he added, “so it’s unlikely you’ll have the partisan organizations stay out.”

“It’s bold and innovative, but, probably, it is not going to work,” said Michael Franz, a professor at Bowdoin College and an expert in campaign finance. “It’s just a matter of time and polling before this thing blows up.”