BOSTON (AP) - Republican U.S. Sen. Scott Brown and his chief Democratic rival, Elizabeth Warren, have signed a pledge to curb political attack ads by outside groups in their Massachusetts Senate race.
Under the terms of the deal, each campaign would agree to donate half the cost of any third-party ad to charity if that ad either supports their candidacy or attacks their opponent by name.
At least one outside group that has targeted Warren immediately raised objections to the deal, while two outside groups that have targeted Brown said they were inclined to respect the deal, with one pledging to suspend its advertising.
Brown first laid out the basic terms of the deal last week, but top-level staffers for both candidates were unable to reach a final deal on Friday.
Then early Monday morning, Warren responded, saying she was ready to sign off on the deal as long as it included a few final changes, such as adding Internet advertising to the deal and closing any other loopholes that could permit third parties to help one campaign or the other by running ads.
Even as she agreed to the deal, Warren conceded she wasn't absolutely sure it would hold.
"Do we know it will succeed? No. But I do know that we go into this in good faith to try to have a chance to make our best case to the voters of Massachusetts," Warren told reporters Monday. "I think that's worth trying."
Brown quickly agreed to the changes, and signed what he described as the "People's Pledge."
"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.
Both campaigns then quickly pointed out that each has already been the target of outside advertising.
The U.S. Supreme Court has paved the way for millions of dollars in spending by super PACs following a trio of decisions capped by the landmark Citizens United case in 2010, which eased restrictions on the use of corporate money in political campaigns.
Referring to Warren's background as a Harvard professor, Brown said "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."
Brown has come under fire from the League of Conservation Voters and the League of Women Voters, which have spent a total of about $3 million on separate ad campaigns criticizing him.
The League of Women Voters' ad rapped Brown for voting with other Senate Republicans to ban the Environmental Protection Agency from controlling gases blamed for global warming. They urged Brown to "protect the people and not the polluters." Another spot by the League of Conservation Voters slammed Brown for siding with "big oil."
The League of Conservation Voters issued a statement Monday saying it was inclined to honor the agreement.
"The only thing oil companies have going for them are their deep pockets, so if this agreement will help sideline them, we welcome it," said Navin Nayak, the group's senior vice president of campaigns. "We hope that Scott Brown will honor his end of the deal when Crossroads and the Koch Brothers inevitably break it."
Warren also pointed to outside ads that have targeted her campaign, specifically Crossroads GPS, an affiliate of American Crossroads, a group with ties to GOP political operative Karl Rove, has already sponsored two ads.
One Crossroads ad used spliced images of Warren with rowdy Occupy Wall Street protesters to claim that she "sides with extreme left" protesters who "attack police, do drugs and trash public parks." A second ad by the group then painted Warren as being too cozy with Wall Street.
The president of American Crossroads, Steven Law, criticized the deal, saying it fails to cover 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," Law wrote.
Warren said she wasn't surprised by Law's comments.
"So let me get this straight. Karl Rove, the king of dirty tricks, doesn't like this agreement?" she said. "Ultimately I don't kid myself about this. The law is what the law is following Citizens United. These groups can legally come in and play these dirty tricks."
The Crossroads GPS ads and the ads from the League of Women Voters and the League of Conservation Voters would all appear to come under the terms of the agreement.
Brown's campaign has also pointed to online ads from Rethink Brown - a political action committee formed last year "to encourage Massachusetts voters to make up their own minds about U.S. Senator Scott Brown's actual record."
The group said Monday it will suspend its advertising as long as other outside groups adhere to the deal.
"Rethink Brown will comply with this agreement between the two candidates, while continuing to educate the voters about the real Scott Brown record," the group said in a statement.
A spokesman for Warren's campaign said joint letters signed by Warren and Brown will be going out to third-party groups - including Rethink Brown and American Crossroads - asking them to pull their ads.
The Senate campaign is expected to be one of the costliest in state history.
Warren has reported raising $5.7 million during the final three months of 2011, eclipsing Brown's $3.2 million for the same period. Brown still enjoys an overall money advantage with $12.8 million in cash on hand, compared to the more than $6 million Warren has in her account.