The campaign of Sen. Jeanne Shaheen (D-N.H.) released a radio ad Monday slamming former Massachusetts senator Scott Brown (R) for declining to sign a pledge to curb the influence of outside groups, a criticism Shaheen has emphasized since Brown announced he would explore a run against her 10 days ago.

"Just a few weeks ago in New York, he was still boasting about his People’s Pledge. But now Scott Brown’s in New Hampshire and refusing to sign the exact same People’s Pledge," says the narrator of the commercial.

When he ran for reelection in 2012, Brown and now-Sen. Elizabeth Warren (D-Mass.) reached an unprecedented agreement designed to dramatically limit outside group advertising in their race. Brown first floated the plan in that campaign, and praised it as recently as last month in an appearance at Cornell University.

But he has declined to sign the pledge since launching his exploratory committee in New Hampshire. Conservative outside groups led by Americans for Prosperity have already spent big in the race, a factor which could explain both Brown's reluctance to sign the pledge and Shaheen's desire for him to sign on.

Shaheen's campaign said it was spending five figures to air the ad statewide. It's her first radio ad of the 2014 cycle.

After telegraphing uncertainty about his political future for months, Brown announced he would explore a bid for the Senate earlier this month, a decision that allows him to raise money for a potential bid. His entrance was good news for Senate Republicans; New Hampshire was not considered a potential pickup opportunity before Brown entered the mix.

"Senator Shaheen's negative ads are a sad commentary on just how out of touch she is with the things people care about," said Brown in response to the ad.