A meeting between campaign representatives for Sen. Scott Brown (R-Mass.) and his Democratic rival Elizabeth WarrenElizabeth Ann WarrenMulvaney aims to cement CFPB legacy by ensuring successor's confirmation Trump calls Nevada Dem Senate candidate 'Wacky Jacky,' renews 'Pocahontas' jab at Warren On The Money — Sponsored by Prudential — Trump floats tariffs on European cars | Nikki Haley slams UN report on US poverty | Will tax law help GOP? It's a mystery MORE over barring outside groups from spending millions in the race ended without a deal.

The two campaign chiefs hit an impasse over loopholes that Warren’s team said were in a proposal put forward earlier in the week by Brown. The deal would have incentivized super-PACs and outside groups to stay out of the race by forcing candidates to donate half of anything outside groups spend on their behalf to charity.

“Elizabeth has been very clear that we need a clean and simple agreement to stop third party advertising in this Senate race,” Warren spokeswoman Alethea Harney said in an email shortly after the meeting concluded. “She is happy that Scott Brown has moved her way on this issue, but unfortunately, the proposal he has put forward still includes loopholes that Karl Rove could drive a tank through.”

Harney added that Warren is still confident an enforceable agreement can be reached and was committed to a final resolution that exceed mere political rhetoric.

The meeting was an unsuccessful conclusion to an unprecedented week of coordination by two political competitors seeking a mutual agreement to diminish the influence of outside groups on campaigns.

Saturday marks the second anniversary of the Supreme Court's Citizens United decision, which cleared the way for outside groups to make unlimited expenditures to influence elections without needing to adhere to the same contribution limits and reporting requirements as official campaigns.

At the crux of the problem is that by law, campaigns cannot coordinate with the outside groups that support them, making it difficult for them to exert influence over the groups even when their involvement is unwanted.

Both Brown and Warren have seen millions of dollars worth of attack ads aired against them by groups aligned with their opponent. Brown has asked groups on both sides to stay out of the race and has called on Warren to say the same for months.

Earlier this week, Warren proposed the two devise an “enforceable agreement” to keep unwanted groups out of their race and suggested their campaign managers meet to hash out the details. Brown’s campaign responded with a proposal dubbed the “People’s Pledge,” but Warren said she preferred the campaigns work toward a consensus in person rather than negotiate through the media.

The details of Warren’s opposition to the proposal discussed Friday were not immediately available, but her statement hinted strongly at concerns she had raised earlier: that by referring only to “independent expenditures” for television and online ads, the agreement left open the possibility that outside groups could air less ambiguous “issue ads” or sink money into the race in other ways without violating the letter of the agreement.

There is little to no precedent for what Warren and Brown have set out to accomplish in their race, one of the most closely watched Senate races of the 2012 cycle.