US Sen. Brown, Warren, agree to curb attack ads Jan 23 12:49 PM US/Eastern BOSTON (AP) - U.S. Sen. Scott Brown and his chief Democratic rival Elizabeth Warren have signed a pledge to curb political attack ads by outside groups. Warren on Monday sent a letter to Republican incumbent Brown urging him join her and sign an agreement to prevent third-party ads. Brown signed the pledge minutes later. Top level staffers for both candidates failed to reach a deal during a meeting last week. Warren in her letter said "we don't want outsiders running this election" and said her agreement sews up loopholes in a proposal Brown made last week. Brown suggested that each candidate donate half the cost of any third-party ad to charity if that ad either supports their candidacy or attacks their opponent. He called the agreement a "great victory" for the people of Massachusetts.