Attorney General Martha Coakley, who only three years ago appeared to be finished politically, will join the race for governor with an announcement in her home town of Medford on Monday followed by an 18-city barnstorming tour for the next three days, a top political aide said.

Coakley, whose campaign will test whether Democrats are ready to take another chance on her leading the ticket next year, will outline a theme of expanding economic opportunity, job creation, and education.

After a lackluster performance that led to her humiliating loss to Scott Brown in the 2010 US Senate race, Coakley is expected to be a formidable candidate in the primary. Recent polls show she is now one of the most popular political figures in the state.

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She joins a Democratic primary field that includes one other woman and four men, including the candidate many analysts sees as her main rival in the early stages of the campaign, state Treasurer Steve Grossman, the former state and national party chairman.

Grossman has been working hard for the past several years, crisscrossing the state and buttonholing party leaders, fundraisers, and activists for support. Coakley, who was expected until this spring to run for re-election, for the most part has not circulated among the party activists.

Still, with her high-profile work as attorney general in the last few years boosting her image, early polls show Coakley leading the pack in the party primary and running the strongest against likely GOP nominee Charles D. Baker. But she faces skepticism within the party rank and file who are concerned about her ability to sustain a strong campaign in the general election.

One of her strongest advantages is support from Democratic women activists and fundraisers, who had strongly encouraged her jump into the race. Until that pressure mounted this spring, members of inner political circles signaled that she would very likely not run, but instead seek a third term as attorney general.

Emily’s List, a Washington-based political group with a mission to elect women to public office, is expected to provide Coakley significant financial resources through its national fundraising network.

The other woman in the Democratic race is Juliette Kayyem, a former Globe columnist who has served as a state and federal homeland security official. Kayyem announced last month she would run, although she had indicated earlier she would not get in the race if Coakley entered the campaign.

Also running is Don Berwick, who ran Medicaid and Medicare in the Obama administration; Joseph Avellone, a biotech executive; and, state Senator Dan Wolf of Harwich, the founder of Cape Air, who suspended his campaign pending his appeal of a State Ethics Commission opinion over his company’s business relationship with Logan Airport.