THE RACE FOR an open state Senate seat in Boston and three adjacent communities was sure to shake up the status quo, with no Italian-American son of East Boston in the race — the profile that has defined the district’s senator for 40 years.

But voters did not seem hungry for too much change, as Joseph Boncore, an Italian-American son of next-door Winthrop, topped a seven-way field to capture the Democratic primary special election.

Joseph Boncore: The sleeper in the race.
Joseph Boncore: The sleeper in the race.

With no Republicans seeking the seat, Boncore is virtually assured of being elected the district’s senator in the May 10 general election.

Boncore captured just over 4,000 votes, according to unofficial figures from various campaigns. Running second, about 400 votes behind him, was former Revere mayor Dan Rizzo.

Boncore rode the wave of a big turnout – and a huge margin – in his hometown, where he won an astounding 65 percent of the vote. Boncore captured about 25 percent of the total vote in the district, which includes all of Winthrop and Revere, East Boston and chunk of downtown Boston neighborhoods, and seven precincts in Cambridge.

Following Rizzo were state Rep. Jay Livingstone, public interest attorney Lydia Edwards, and former State House aide Diana Hwang. Revere City Councilor Steve Morabito and East Boston business owner Paul Rogers finished well back in the voting.

Boncore, an attorney and elected member of the Winthrop Housing Authority, was pegged by insiders early on as the sleeper in the race. While much of the political chatter centered on Rizzo’s political comeback effort – he lost the Revere mayor’s seat last fall — and the scramble among Boston candidates, which included two women with compelling backgrounds, Boncore kept his head down and his eyes on the prize.

His campaign was busy organizing his neighbors in tight-knit Winthrop to turn out in droves for a favorite son, while reaching into other parts of the district to sway enough voters to put him over the top.

One precedent for Boncore’s victory may be a Congressional contest from nearly two decades ago.

In 1998, when Joe Kennedy II — the father of the current 4th District congressman — gave up his Boston-based House seat, a 10-candidate Democratic free-for-all ensued. The primary featured several well-known names, including former Boston mayor Ray Flynn and ex-state senator George Bachrach.

But it was then-Somerville mayor Michael Capuano who snuck past them all by piling up an enormous margin in his hometown, while others were fighting it out across the rest of the district.

As with Tuesday’s state Senate race, winning the Democratic primary was tantamount to winning the congressional seat in the overwhelmingly Democratic district. Capuano effectively captured the seat by carving out 23 percent of the primary vote, very close to Boncore’s 25 percent share in the First Suffolk and Middlesex District Senate primary.

In the last two state primary elections in the Senate district, in 2012 and 2014, about 45 percent of the Democratic primary vote came from Boston, 28 percent came from Revere, and Winthrop and Cambridge each accounted for about 13 percent.

Those vote shares shifted significantly in Tuesday’s balloting, in ways that point clearly toward the final outcome.

Based on unofficial results, Boston residents accounted for only 35 percent of the 16,677 votes cast, a 10 point drop. The Cambridge share also declined, down to 10 percent of the vote. Meanwhile, Revere’s share of the vote jumped from 28 percent in the last two Democratic primaries to 34 percent on Tuesday.

But it was Winthrop that showed the biggest gain, an 8 point increase from 13 percent to 21 percent of the total primary vote. Just as important was the huge margin Boncore ran up in Winthrop, where he captured nearly two-thirds of the vote.

Rizzo, who lost a bitter mayoral reelection contest last fall, was unable to command that outsized level of support in his hometown, winning 49 percent of the Revere vote.

Livingstone, Edwards, and Hwang were pegged as the progressive flank of the field, with Rizzo and Boncore seen as more moderate Democrats. But the race wasn’t heavily defined by issues.

Charter schools, which are being debated on Beacon Hill and are likely to be the subject of ballot question this fall, were one issue distinguishing the candidates. Boncore, who was endorsed by the Boston Teachers Union, opposes any increase in the cap on charters, while Rizzo voiced support for raising it.

Rizzo was aided by about $42,000 in independent campaign spending by Democrats for Education Reform, a pro-charter organization.

He was also on the receiving end of a flurry of last-minute attack fliers sent by a labor-backed PAC that accused him of endorsing Republicans in several high-profile races. Rizzo denied ever endorsing a Republican for office, and has filed a complaint with the state Office of Campaign and Political Finance charging that the PAC failed to disclose its $7,400 expenditure on the mailings within 24 hours, as required.

The race to fill the seat vacated in January by Anthony Petruccelli featured a slew of candidate forums, but it proved difficult to draw attention to the election. First-time candidate Diana Hwang, short on name recognition but deep in campaign cash, took to the airwaves with a cable television ad, an unusual move in a state Senate race.

But this was always a race that was going to be won on the ground, and the candidates with established bases of support and well-oiled campaign operations from past contests had a huge leg up. That put Boncore, Rizzo, and Livingstone in the top tier from the start.

Edwards and Hwang ran strong races for first-time candidates, and they won over the city’s newspaper opinion pages, even if not enough of the district’s voters. Edwards got the Boston Globe’s endorsement, while Hwang was endorsed by the Boston Herald.

Livingstone, who represents Beacon Hill and parts of Cambridge, might have been hurt by Hwang and Edwards cutting into the base of progressive-leaning voters who may otherwise have veered his way.

By the same token, though, either one of them might have had a clearer path to victory without him in the race.

[This was revised to reflect updated vote totals from Revere. Dan Rizzo won 49 percent of the Revere vote, not 40 percent as originally reported.]