Maura Healey, the attorney general of Massachusetts, announced on Thursday that she was running for governor.

Ms. Healey, a Democrat, is the most well known of the candidates who have entered the race to replace Gov. Charlie Baker, a Republican who is not running for re-election.

“I’ve stood with you as the people’s lawyer, and now I’m running to be your governor to bring us together and come back stronger than ever,” Ms. Healey said in her announcement video.

Her opponents in the primary will include Sonia Chang-Díaz, a progressive state senator, and Danielle Allen, a Harvard professor who received a MacArthur genius grant for her work on race and political theory, entered the race in June.

On the Republican side, former President Donald J. Trump endorsed Geoff Diehl, a former state lawmaker.