Mass. political rivals set to halt attack ads

Early Monday morning, Warren responded, saying she was ready to sign off on the deal as long as it included 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 said Monday. "I think that's worth trying."

Brown quickly agreed to the changes, and signed what he described as the "People's Pledge."

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 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." The League of Conservation Voters issued a statement Monday saying it was inclined to honor the agreement.

Warren 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.