Updated

Martha Coakley, the popular Democratic state attorney general who lost the 2010 U.S. Senate special election to Republican Scott Brown, is joining the race for Massachusetts governor, her campaign announced Sunday.

Coakley planned a formal campaign announcement Monday morning in her hometown of Medford, followed by a three-day blitz of 18 cities and towns. She intended to listen to voters and discuss her vision for strengthening the state's economy and improving its education system, her campaign announced.

Coakley, 60, didn't immediately return a message seeking comment Sunday.

She's joining a field that has become crowded since Democratic Gov. Deval Patrick announced he wouldn't seek a third term next year.

The other Democrats already in the race include state Treasurer Steven Grossman, former Obama administration health care official Don Berwick, former federal and state homeland security official Juliette Kayyem and former Wellesley selectman Joseph Avellone.

The candidacy of another Democratic gubernatorial hopeful, state Sen. Dan Wolf, is pending the outcome of discussions with the state Ethics Commission over his ownership stake in Cape Air.

Democrats also are awaiting the decisions of U.S. Rep. Michael Capuano and Somerville Mayor Joseph Curtatone.

Republican Charles Baker is the only GOP candidate to declare for the governor's race.

Coakley's loss to Brown in the 2010 Senate race was a stunning upset that rocked both the Democratic Party and the Obama administration, who viewed the seat as safe for Democrats and Coakley as the pivotal 60th vote to preserve the Democratic "supermajority" in the Senate.

Coakley also faces an unusual political hurdle in Massachusetts political history -- the "curse" of the attorney general's office.

Since 1958, five former Massachusetts attorneys general have sought the governor's office. All five -- George Fingold, Edward J. McCormack Jr., Francis X. Bellotti, Scott Harshbarger and Tom Reilly -- failed, either by losing their party's primary, losing the general election or, in the case of Fingold, dying before Election Day.