Newton Mayor Setti Warren (D) announced his Senate bid Monday, taking direct aim at Sen. Scott Brown (R-Mass.) in a lengthy announcement video posted on his campaign website.

"I'm probably about as well known as Scott Brown was at this point two years ago," said Warren, an Iraq war veteran and the state's first popularly elected black mayor. 

Warren calls Brown "an honorable man," in the video, but says "he has not been the independent voice in the Senate that so many expected him to be."

Warren is the third Democrat to get in the race against Brown, joining businessman Alan Khazei and activist Robert Massie. 

National Democrats have pegged Brown as their top Senate target in 2012, but the race against him still lacks a high-profile candidate. Brown is also sitting on a campaign war chest of more than $8 million.