Warren will formally declare she's running on Wednesday, Kyle Sullivan, a Warren spokesman, said. She plans to greet commuters in Boston and make other stops during the day across the state.
"The pressures on middle-class families are worse than ever, but it is the big corporations that get their way in Washington," Warren said in a statement released Tuesday. "I want to change that. I will work my heart out to earn the trust of the people of Massachusetts."
Democrats have been seeking a major challenger for the seat long held by the late Sen. Edward Kennedy. Democrats hoping to keep their narrow Senate majority have made Brown a top target in the blue state. Kennedy's former seat has special significance for Massachusetts Democrats. Warren will join a crowded primary field but was heavily courted to join the race.
Warren is a Harvard Law professor who was tapped by President Barack Obama last year to set up a new consumer protection agency, but congressional Republicans opposed her becoming the director. She worked to set up the agency before returning to Massachusetts this summer.
Warren, 62, is a favorite of liberals and consumer groups, but some Democrats, including Boston Mayor Thomas Menino, have voiced skepticism about how strong a candidate she will be, given her lack of political experience.
Republicans have already branded Warren as a liberal academic whose Harvard ties put her out of touch with the concerns of working families. They've also mocked her as an outsider whose roots are in Oklahoma, not Massachusetts.
Democratic leaders, however, said her national profile would help her raise the money needed to topple Brown, who has more than $10 million in his campaign account. Democrats contend that while Brown has strong ties to Wall Street and other powerful financial interests, Warren's long career as a consumer advocate offers a striking contrast for voters who care deeply about jobs and the ailing economy.
A recent Boston Globe poll showed Brown as the most popular major politician in the traditionally Democratic state. Brown shocked the political establishment by beating Martha Coakley in last year's special election to succeed Kennedy. He was a little-known state senator who cast himself as a moderate, an average guy with his trademark barn coat and pickup truck, even once posing as a Cosmopolitan magazine centerfold.
Coakley, the state's attorney general, was widely seen as an early favorite, but she ran a lackluster race. She famously mocked Brown for greeting voters outside Boston's Fenway Park in freezing weather, a gaffe that cost votes because she was seen as taking the race for granted.
Warren has spent the past several weeks meeting with party activists and voters across the state as part of what she called a listening tour. She's already gotten a boost from EMILY's List, which raises money for female Democratic candidates. The Progressive Change Campaign Committee, a national liberal group, has been raising money and seeking campaign volunteers for Warren for weeks.
Other Democrats already announced include Swetti Warren, no relation to the consumer advocate, the first-term mayor of the affluent Boston suburb of Newton and the state's first popularly elected black mayor; City Year youth program co-founder Alan Khazei; immigration attorney Marisa DeFranco, state Rep. Tom Conroy and Robert Massie, who unsuccessfully ran for lieutenant governor.