Ayanna Pressley defeats 10-term incumbent Mike Capuano in Mass. Democratic primary

WASHINGTON — A month ago, Ayanna Pressley told NBC News she was in "a fight for the soul of our party." On Tuesday night, she won it.

In a stunning upset in John F. Kennedy's old congressional district, Pressley, the first woman of color elected to Boston's City Council, defeated Rep. Mike Capuano, D-Mass., a 10-term incumbent in a high-profile Democratic primary.

Polls had shown Capuano, who has never faced a serious challenge since being elected in 1998, with big leads. But Pressley defied the odds to not just win, but outpace Capuano so decisively that he conceded the race less than an hour and a half after polls closed.

With about a third of precincts reporting, Pressley had 53 percent of the vote to Capuano's 47 percent, or 16,429 votes to 14,379.

"Clearly the district wanted a lot of change," Capuano told supporters. "The district just is very upset with lots of things that are going on. I don't blame them. I'm just as upset as they are. But so be it, this is the way life goes."

"Ayanna Pressley is going to be a good congresswoman and I will tell you that Massachusetts will be well served," the defeated congressman said.

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Massachusetts' 7th congressional district is the only one in New England where racial minorities outnumber whites and Pressley argued she could better represent the Boston and Cambridge-area district than Capuano, who is white.

It's another victory for the insurgent left, which has seen its biggest successes when people of color embrace progressive ideology, such as with Florida gubernatorial candidate Andrew Gillum and Alexandria Ocasio-Cortez, who vocally supported Pressley.

"I'm excited for her. I'm really feeling enormous sense of pride for what's she's accomplished," former Massachusetts Senator John Kerry on MSNBC Tuesday night. "I'm happy for Massachusetts."