Boston • The day before Ayanna Pressley rattled national Democrats by overwhelming Rep. Mike Capuano, a popular 20-year incumbent, in Tuesday’s primary, she brought her youthful canvassers to the campaign’s Jamaica Plain headquarters to fire them up for the offensive to come.

Pressley always delivers fire in public speeches, but the 44-year old Boston city councilor is careful and quietly analytical in dissecting electoral imperatives. In the midst of her oration, she explained her coming victory in one sentence.

"The district has changed," she said, "the needs have changed, and given what's happening in Washington, the job description has changed."

Her volunteers needed no prompting to chant her campaign's killer slogan, "Change Can't Wait." They wouldn't have to wait long.

Two hours earlier, a couple dozen Capuano supporters had gathered on the front porch of a supporter's home a few miles north in the working class city of Everett.

Capuano, 66, did not offer a stemwinder. That's not his style. He simply wanted to say thank you to longtime friends he was counting on to bring out his voters one more time.

"You guys have done a fantastic job," he said. "You've welcomed me into the community, as I knew you would. ... And I've tried to return the love and the respect and the appreciation to the best of my ability ... I really expect that we'll do as well here as we do anywhere and it's because of you."

Capuano was right about Everett — it was the one jurisdiction he carried overwhelmingly. But his 65 percent share there was far less than hoped for, and on election night, he ended up conceding long before the media called the race. His hometown of Somerville, where he had served as mayor, gave him a paltry margin of just over 100 votes. He knew that was the end.

Pressley didn't just win. She swamped Capuano, 59 percent to 41 percent. The size of the margin seemed to shock even the victor, as suggested by a widely circulated video of her tearful, stunned elation at the first word of her triumph.

Pressley's surprise success has been compared to Alexandria Ocasio-Cortez's defeat of Democratic Rep. Joseph Crowley in New York earlier this summer, and there are certainly commonalities.

As Pressley noted, Massachusetts' 7th Congressional District was not the same place that first elected Capuano in 1998. It was now majority-minority — it was drawn to be friendly to minority candidates. Many of its previously working-class areas, including Capuano’s own Sommerville, have drawn in younger professionals with no ties to the old white ethnic politics that the late House Speaker Tip O’Neill, who represented the area, knew so well.

At Pressley’s victory party, Julia Kehoe, 25, Clare Abreu, 34 and Dan Babek, 34, were elated at what their canvassing had wrought, and they were part of the new 7th District, working, respectively, on energy efficiency, biophysics and biotech.

And Pressley spoke for, as she often put it, the "communities that far too often go unseen and unheard." Tanisha Sullivan, president of the Boston NAACP, said in an interview that for minority communities, Pressley's victory "means that finally, we're speaking for ourselves."

But there were important differences between Pressley and Ocasio-Cortez. Pressley has deep roots in local politics, having worked for, among others, former Sen. John Kerry, with whom she remains close. In 2016, Pressley endorsed Hillary Clinton, not Bernie Sanders.

What should trouble Democratic incumbents is that Capuano lost even though the district's liberal electorate saw nothing really wrong with him. A member of the House Progressive Caucus, he has backed single-payer health care longer than Pressley. He also took her challenge seriously from the start.

But long service was no match for a charismatic candidate who said a district as progressive as the 7th should expect more activist representation and a more adventurous approach to politics.

Pressley's feat is likely to be replicated only in safe Democratic seats with racially diverse makeups. But there are quite a few places like that, and more in which the party's primary voters are eager to nominate women.

And Democrats everywhere need to take account of an argument she reiterated in her victory speech: that the problems of racial and economic inequality predate the rise of Donald Trump.

Being against Trump will likely rally Democrats in November's elections, but many in the party are not interested in a simple return to the pre-Trump status quo. "Change Can't Wait" worked because it brought to life their sense of urgency.

E.J. Dionne

E.J. Dionne writes about politics in a twice-weekly column and on the PostPartisan blog. He is a government professor at Georgetown University, a senior fellow in governance studies at the Brookings Institution and a frequent commentator on politics for National Public Radio and MSNBC. He is most recently a co-author of “One Nation After Trump.” ejdionne@washpost.com. Twitter: @EJDionne.