The Massachusetts race is already a national affair, with outside groups on the left and right lining up to weigh in. But as public awareness and skepticism about money in politics rises, both candidates declared they wanted super PACs and other third-party committees to close their wallets.
"This historic agreement means the candidates will be in control of their own campaigns and accountable for what is said," Brown said in a statement on Monday.
Warren, in a letter proposing the final draft, said the pledge would "give the people we want to represent an opportunity to make a choice free from the kind of out-of-control, outsider advertising that has plagued so many recent elections."
While nothing new to campaign season, third-party spending is playing a newly critical role in the GOP presidential primary, thanks in part to the rise of super PACs and the Citizens United Supreme Court ruling. Some GOP candidates have disavowed the resulting barrage of negative advertising and called for its removal. But the complaints are hollow, since candidates do not control the committees and, by law, are not allowed to coordinate with them.
Warren and Brown said they believed they have found an enforceable solution. Under the agreement, if an outside group buys advertising supporting a candidate, that candidate has to pay half the cost of the advertising to a charity of the opponent's choosing. The same is true for advertising opposing a candidate.
"In short, your spending will damage the candidate you intend to help," Warren and Brown wrote in a form letter to be sent to third party groups.
Even as it was announced, Brown pointed to two groups running ads against the senator. "The Brown campaign said it expects the ads will come down in short order," the campaign said Monday.