The ad was announced the same day Brown and his top Democratic rival, Elizabeth WarrenElizabeth WarrenSenate committee clears Carson nomination AT&T beefs up lobbying after merger proposal Sanders: I'll work with Trump on trade MORE, signed a groundbreaking agreement to discourage outside groups and super-PACs from spending millions of dollars on ads in the race.

Warren launched her first ad in November, pushing back on negative ads aired against her by conservative group Crossroads GPS.

Listen to the ad: