Pressley, who will not face a Republican challenger in November, becomes the first African-American woman elected to Congress from the Commonwealth.
The race between Capuano and Pressley was perhaps the most closely watched contest in MA, especially since Pressley drew comparisons to Ocasio-Cortez.
During the campaign, Capuano touted his credentials as an "unabashed liberal", but he found himself competing with Pressley over who was the more progressive candidate.
The Dorchester resident Carla Monteiro, 35, voted for Pressley because, she said, she felt a strong connection to her history, and felt Pressley had a record of making change.
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A part of the wave of women of color breaking into traditionally white positions of power across the country, Pressley's win echoed that of Alexandria Ocasio-Cortez's over the longtime representative Joe Crowley in NY, and the ascent of Stacey Abrams in Georgia, who - if she wins - will become the first African American female governor in United States history.
However, there are significant differences between the two races. Pressley acknowledged that she and Capuano would probably vote the same way on most issues, but she has argued that she could lead "a movement" from the seat. No Republicans were running, so only a write-in campaign in November could possibly stand between her and Washington.
The 7th District encompasses Boston's residential neighborhoods and its surrounding suburbs, with large Hispanic, black and Asian populations, and is the state's only majority-minority district.
Quentin Palfrey has won the Democratic primary for lieutenant governor in MA, ending comic Jimmy Tingle's bid to shift from political humor to political office.
Democratic U.S. Sen. Elizabeth Warren sailed through her primary unopposed. He also had the support of the Congressional Black Caucus.
Rep. Stephen F. Lynch easily won his Democratic primary Tuesday in his bid for a ninth full term.