Ayanna Pressley, 44, beat veteran politician Mike Capuano, 66, after mounting an insurgent campaign in MA 7th Congressional District.
Pressley is likely to become the first black woman elected to Congress from MA in November, since there are no Republican candidates in the race.
Pressley was endorsed by fellow congressional upstart Alexandria Ocasio-Cortez.
Pressley was long considered the underdog.
That wave of change sweeping Pressley into Congress could help the incoming freshmen overcome her low spot on the political totem pole, said Democratic strategist Scott Ferson, who helped Seth Moulton win over incumbent U.S. Rep. John Tierney in 2014.
Pressley, who carried endorsements from local Democrats like Attorney General Maura Healey and Boston City Councilor Michelle Wu, should have an easy time making friends, despite not having an endorsement from any of the state's congressional delegation or the Congressional Black Caucus. Her victory comes just two months after Alexandria Ocasio-Cortez similarly defeated a top House leader in a primary for a NY congressional seat. But she made her background a major part of the race, highlighting the importance of boosting diversity in the Democratic Party, talking about her own experience with sexual assault, and regularly calling for more "bold, activist leadership".
But both Pressley and Capuano admit there's likely no difference between them in terms of the way they'll vote in Congress, and moreover, neither one has expressed outright opposition against Democratic leader Nancy Pelosi.
"The Democratic base is extremely diverse and the representatives they have in Congress are not almost as diverse as the constituency", said Joshua Dyck, a professor of political science at the University of MA at Lowell.
Democrats need to pick up 23 seats to take a majority in the House of Representatives, and two to take control of the Senate, if they are to serve as an effective foil to Trump's agenda.
Capuano was endorsed by prominent MA figures, including former Gov. Deval Patrick, the state's first black governor. She also worked for former senator John Kerry. She ousted 10-term New York Rep. Joe Crowley in a Democratic primary in June.