Democratic activist Alan Khazei announced Tuesday that he will vie for his party's nomination to face Massachusetts Sen. Scott Brown (R). 

Khazei, co-founder of the nonprofit group City Year, posted the announcement on his website and said he is holding a kickoff event in Boston.

"Today I’m announcing that I am an official candidate for U.S. Senate in Massachusetts in 2012," he wrote. "We have to grow our economy, create new jobs, and expand opportunity for the middle class. Together, we can do it."

Khazei, who ran in the Democratic primary in 2009 to face Brown, is the third member of his party to jump into the race. But Democrats are still looking for a top-tier candidate to face the freshman GOP senator.

Minister and activist Bob Massie and attorney Marisa DeFranco have also announced they will seek the Democratic nomination. 

Brown shocked political observers by winning a special election to fill the seat of the late Sen. Edward Kennedy (D-Mass.) in January 2010. He has already started filling his campaign coffers and hiring staff in anticipation of a highly-contested reelection race next year.