"We won? we won?" she repeats as she looks around the room.
Garrison served one term as a state representative in 1992, but she has lost countless elections since leaving that office.
Before becoming a councillor she worked for Senator John Kerry and Congressman Joe Kennedy - whose grandfather, John F Kennedy held the seat Ms Pressley is set to win before he became president.
They moved to Brooklyn, and Pressley attended Boston University from 1992 to 1994, but left school to work full time at a hotel to support her mother.
Ms. Pressley, the first black woman elected to the Boston City Council, challenged Mr. Capuano from the left and on style and identity, in a pattern similar to the upset victory of Alexandria Ocasio-Cortez earlier this summer in NY.
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But analysts still say the United States appears resigned to the likelihood of a final military victory by Syrian regime forces. President Trump tweeted yesterday addressing to not attack Idleb because the operation could leave hundreds of people dead.
In challenging Capuano, who had not faced a primary challenge since he was first elected in 1998, Pressley argued that she was more attuned to the needs of residents of the state's only congressional district where a majority of residents are not white.
Pressley had the endorsement of Ocasio-Cortez, whose victory in NY this summer prompted Democratic National Committee chairman Tom Perez to declare her "the future of our party", as the party shifts leftward from the watery managerialism of Hillary Clinton and her ilk.
The race drew national attention because it was seen by some as a possible indication of deeper generational and philosophical shifts within the Democratic Party, led by younger candidates who take more liberal positions on issues ranging from universal health care to immigration enforcement.
Only two of Massachusetts's nine House members are women, and one is retiring.
But when you put the Pressley upset together with the defeat of a couple of veteran members of the House leadership on Beacon Hill, the chant at Pressley headquarters Tuesday night may be prescient - "change can't wait".
The race was just one of a slate of contested primaries in MA on Tuesday. Kennedy, grandson of the late Sen. Pressley will run unopposed in November, giving her a virtual lock on the seat - the first in Congress that will be held by a black woman from MA. That race remained too close to call Wednesday. In 2009, she was the first woman of colour ever elected to the Boston City Council.