As outside “dark money” groups pump cash into the race, former U.S. Sen. Scott Brown is spurning the “People’s Pledge” on outside campaign spending that he championed in 2012.

“Sen. Brown repeatedly took credit, and deservedly so, when the Massachusetts People’s Pledge reduced the impact of out-of-state groups, funded by anonymous donations,” said Common Cause President Miles Rapoport. “Brown’s race against Elizabeth Warren in 2012 was a model for what can happen when candidates negotiate in good faith. We’re perplexed by his apparent refusal now to even discuss a similar pledge in his contest this year with Sen. Jeanne Shaheen.”

“In the 2012 New Hampshire gubernatorial race, unaccountable outside groups outspent candidates four-to-one. This is no way to run a democracy, said Public Citizen President Robert Weissman. “The people of New Hampshire deserve better in this year’s Senate race. They need Scott Brown to join Sen. Shaheen in pledging to reject outside spending.”

The People’s Pledge is an agreement in which candidates promise to make charitable contributions from their campaign treasuries to offset spending on their behalf by “independent” super PACs and nonprofit organizations. In Massachusetts, the agreement between Brown and Warren helped limit spending by outside groups to only 9 percent of the overall campaign total. That’s a fraction of what those groups put into contested races elsewhere.

“We joined last month with our colleagues at Public Citizen in urging Sens. Brown and Shaheen to negotiate in good faith on a New Hampshire version of the pledge,” Rapoport said. “Sen. Shaheen has endorsed the People’s Pledge and indicated her willingness to discuss a similar arrangement in New Hampshire, but Sen. Brown appears uninterested.”

As recently as February however, in a speech at Cornell University, Brown hailed the success of the pledge in Massachusetts. “I came up with the idea actually in the last election,” he told students. “We didn’t need another 30 to 40 million dollars coming in to distort our records and positions on things, so … what we came up with was the People’s Pledge.”

Common Cause and Public Citizen also called on Shaheen to withdraw a radio advertisement attacking Brown on the issue. “We were disappointed that Sen. Shaheen chose to keep airing the ad. Withdrawing it would have been a good faith gesture on her part to open negotiations,” Rapoport said.

Common Cause and Public Citizen urge other prospective New Hampshire Senate candidates, as well as congressional candidates across the country, to join in the pledge.

Outside organizations already have put nearly $85,000 into the U.S. Senate contest in New Hampshire, according to the Center for Responsive Politics, a nonpartisan organization that tracks campaign spending reports.