This morning, Elizabeth Warren and Scott Brown announced that they had hammered out an agreement to put up a dam against the flood of outside cash that's expected to gush into their Senate race. Even if the dam ends up springing a few leaks, they deserve credit for trying. Good for them.

The deal wasn’t easy to hammer out, because campaigns don’t have any power to restrict third-party-group spending. But the arrangement deals with this problem in an artful way:

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.

Warren first floated the idea of an enforceable truce; Brown laid out the terms of the deal last week; Warren responded this morning that she wanted a few final changes, such as tightening up a few loopholes that third party groups might exploit; Brown quickly agreed.

Obviously it would seem borderline impossible to get a group like the Rove-founded Crossroads GPS to refrain from running ads in the state. The group has already invested hundreds and hundreds of thousands of dollars in multiple spots that were not only dishonest, but actually contradicted each other in a comically hapless way. The beauty of this arrangement is that if Crossroads runs more ads in the state, Brown’s campaign will, in theory at least, be penalized for it.

The Crossroads empire, as it happens, already appears to be signaling that it may not honor the deal:

The president of American Crossroads, Steven Law, quickly criticized the deal, saying it fails to cover union phone banks, direct mail, and get-out-the-vote drives, “all union core specialties.”

This seems like a pretty weak objection; what’s to stop national conservative and Tea Party groups, which also do this kind of organizing regularly, from mobilizing on Brown’s behalf? Nothing. Indeed, Brown’s special election victory in 2010 was fueled by this type of politicking. It seems Crossroads may be laying the groundwork for breaking the agreement.

By contrast, the League of Conservation Voters, a liberal third party group that had been spending heavily on Warren's behalf, quickly issued a statement supporting the arrangement.

I asked Public Campaign Action Fund’s David Donnelly for comment on the deal. He emailed: “It’s good for them, it’s good for voters, and it’s a good model for every competitive race in the country.”

Agreed. And in political terms, we’re in uncharted waters: Could this possibly succeed in muzzling the Crossroads operation? If not, how will the politics of it play? Will other campaigns emulate this approach? It’s another reason the Massachusetts Senate race will — and should be — one of the most closely watched Senate races in the country.