Montana Senator Jon Tester wrote an open letter this week to the man challenging his re-election, Congressman Denny Rehberg, encouraging a truce much like the one brokered in the Massachusetts Senate race between Elizabeth Warren and incumbent Scott Brown. But will shady third-party groups funded by big business and orchestrated by Karl Rove play ball?

“Let’s reject and work to keep all third-party radio ads about you and me out of Montana,” wrote Tester. “Let’s reject efforts by outside groups to undermine Montana’s tradition of elections decided by people — not corporations.”

A Rehberg spokesman responded by saying, “This is certainly an interesting proposal by Sen. Tester. We are going to give it a close look and we will respond in due course.”

The deal in Massachusetts — borne perhaps from that Cold War-era sensibility that neither side benefits from assured destruction — requires the candidate who benefits from an expenditure by an outside group to donate half the sum to a charity of their opponent’s choice.

But the calculus might be different in Montana, where Tester has the larger traditional campaign warchest (over $3.8 million raised already) and Rehberg could be more dependent on outside help from groups like Karl Rove’s Super PAC, American Crossroads.

This bears watching closely, as Rehberg has tacked to the center by, for instance, voting against Paul Ryan’s Medicare privatization scheme when it came up for a vote in the House. Campaign finance has become an increasingly salient issue for the public, and there are big risks to appearing overtly cynical about money in politics.