NASHUA, N.H. — Democratic Sen. Jeanne Shaheen is calling on newly announced Republican challenger Scott Brown to sign the same pact in New Hampshire that helped prevent outside groups from pouring millions of dollars into his last Massachusetts Senate election.

In a letter sent Saturday, less than 24 hours after the former U.S. senator from Massachusetts entered the race, Shaheen said she “very much admired the People’s Pledge” that Brown signed with Sen. Elizabeth Warren in 2012. Brown lost his Senate seat to Warren.

“I believe it limited the influence of outside groups and allowed the people’s voices to be heard,” Shaheen wrote. She asked Brown to make a similar pledge to give New Hampshire voters “the assurance that their voices will not be drowned out by third-party expenditures.”

Brown would not comment on whether he would agree to a pledge but said Saturday that Shaheen is on a multiple-city West Coast fundraising swing that he says will provide money to third-party groups for more outside negative ads against him.

“It’s hard to view Jeanne Shaheen’s actions as anything other than hypocritical and self-serving,” Brown said.