"Change is coming and the future belongs to all of us", Pressley told wildly cheering supporters Tuesday night. "I'm sorry it didn't work out, but this is life".
She will likely become the first African-American from MA elected to Congress this November, as there are no GOP lawmakers running in her district.
Ms Pressleyargued she would bring new energy and awareness of the needs in the state's only congressional district where a majority of residents are not white.
Pressley may be considered an outsider by virtue of who she is. At Quincy's Thomas Crane Library, 331 voters had cast a ballot with two hours to go before the polls closed, putting the polling placewell on track to surpass the 375 voters that Warden Marion Flavin said it sees in during a typical primary.
"I'm sorry it didn't work out, but this is life, and this is O.K. America's going to be O.K. Ayanna Pressley is going to be a good congresswoman, and I will tell you that MA will be well served", Capuano said, according to The New York Times.
The 7th District is heavily Democratic, and Pressley is now expected to coast to victory in November. She'll face Geoff Diehl, a state representative who served as co-chair of Trump's 2016 campaign in the state and defeated two other Republicans for his party's nomination. She was the first black woman elected to the Boston City Council in 2009.
Pressley's message wasn't just one for the Republican Party, whose leadership structure is largely older, whiter and more male than the electorate.
Her total in Boston was just 1,800 fewer votes than Capuano received throughout the whole 7th district.
Governor Charlie Baker, a Republican who regularly shows up in opinion polls as one of the most popular US governors, is also expected to be re-elected.
"With so much at stake in the era of Trump, tonight's results make clear what Ayanna Pressley knew when she boldly launched her campaign against a ten-term incumbent: Change in the country and Congress can't wait", said Jim Dean, chair of the liberal group Democracy for America.
Pressley will vie for a seat in the House of Representatives after a career in Boston's City Council.
Or, as Pressley put it in her victory speech: "It's not just good enough to see the Democrats back in power". Bernie Sanders (D-Vt.), she later supported it. Capuano also backed Sanders' quixotic health care scheme, which would cost approximately $32.6 trillion, according to Mercatus scholar Charles Blahous.
Massachusetts' last Democratic primary upset came in 2014, when Seth Moulton defeated Rep. John Tierney in the state's 6th Congressional District.