In a campaign shift, Massachusetts Senate candidate Elizabeth Warren (D) is taking direct aim at Sen. Scott Brown (R).

Her latest ad is set in a Lowell boxing gym, where trainer Art Ramalho calls Warren a "real fighter." Brown, Ramalho says, has "been siding with the big money guys. ... I think Scott Brown is with the big corporations. Elizabeth is different."

Until now, neither candidate in the high-profile race had gone explicitly negative. But Warren has been under pressure from fellow Democrats to change her strategy who see her past ads as ineffective. Despite Democrats' demographic advantage and Obama's popularity in the state, polls suggest the personally-liked Brown has a slight lead.