NEWTON — After a brief break from the stage, Camelot is back for another act.

Joseph P. Kennedy III won a decisive victory Tuesday over Republican Sean Bielat in the race to fill the open congressional seat left vacant by the retirement of US Representative Barney Frank.

With 90 percent of the vote counted, Kennedy, 32, the son of Joseph P. Kennedy II, the former US representative, and grandson of the late US senator Robert F. Kennedy, led with 61 percent of the vote in the redrawn Fourth Congressional District, which extends from Newton to Fall River.

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As the race was called for the Democrat, cheers spread among supporters gathered at the Newton Marriott for a campaign party. When Mayor Setti Warren of Newton introduced Joseph Kennedy as the region’s next congressman, people exchanged high-fives and chanted, “Let’s go Joe.”

Kennedy’s win restores his family’s long connection to the nation’s capital, which dates to 1947, when 29-year-old John F. Kennedy became a Massachusetts congressman. For the next 64 years, at least one member of the Kennedy family served in the House, Senate, or White House. The family’s streak was broken in 2011, with the retirement of US Representative Patrick Kennedy of Rhode Island, son of Senator Edward M. Kennedy of Massachusetts. Edward Kennedy died in office in 2009.

“This is an incredible moment for me, and I can’t think of anyone I’d rather share it with than you,” Joseph Kennedy told his supporters.

He later added, “I’m humbled, I’m grateful and I’m ready to get to work.”

The cover band at Bielat’s event cut short their rendition of The Beatles’ “Come Together” as the Republican, joined by his wife and two young children, entered the room of supporters to concede the election.

“The night has not developed as we had hoped,” Bielat said, never letting his smile slip. “We ran the campaign we wanted to run.”

Kennedy will join at least seven other Democrats as a member of the state’s delegation to the House of Representatives. The only congressional race still undecided was the fiercely fought contest between Republican Richard Tisei and incumbent Democrat John Tierney in the Sixth District, which was too close to call at midnight.

The delegation shrank by one this year, to nine members, following redistricting. Also exiting the delegation, in addition to Frank, is retiring US Representative John Olver, whose Western Massachusetts district was absorbed into another as part of redistricting.

Kennedy, a former Peace Corps volunteer and county prosecutor, had for several years been considered a potential candidate for office.

“The tug of public service has always been part of the Kennedy life,” said Paul Kirk, a longtime Kennedy family friend who was appointed temporarily to the Senate after Edward Kennedy’s death. In an interview, Kirk said Kennedy will enter Congress “bearing the name proudly but without any sense of entitlement.”

Kennedy had to wait an hour in line to vote for himself Tuesday morning, due to long lines at his polling place in Brookline, the Kennedy campaign said. Kennedy and his fiancée, Lauren Anne Birchfield, chatted with other waiting voters and then cast their ballots around 8:30 a.m.

Julian E. Zelizer, professor of history and public affairs at Princeton University, said that Kennedy’s election is “important for nurturing the legacy” of the Kennedy family.

“And it matters to the Kennedys,” he said. “It’s not just about fame. This is a political family.”

As a freshman congressman with a famous name, Kennedy is likely to keep a low profile at first, said Zelizer.

Patrick Kennedy said in an interview, “I joke with Joe that I did all I could to lower the bar for him in Washington, so he’ll look like a superstar.”

The former representative, an advocate for brain science research, said that “all my family feels the same way” about Kennedy’s victory: “He’s carrying the water for all of us in terms of enhancing our ability to advocate for the causes that are important to each of us.”

For Bielat, the race marks his second loss in two years. The Marine Corps Reserve officer, 37, gave Frank a tough race in 2010.

Asked if he planned to run for office again, Bielat demurred. “That’s like asking a woman walking out of a delivery room if she’s going to have another baby,” he said in an interview after his speech. “We’ll stay involved, but we’ll see longer term.”

Kennedy widely outspent Bielat in 2012, raising more than $4 million to Bielat’s $866,000, according to mid-October numbers compiled by the Center for Responsive Politics.

During the campaign, Bielat accused Kennedy of trying to coast to the office on his family name. The attack resonated with some voters, who said they are tired of the state’s famous political dynasty.

“I’m so fed up with the Kennedys,” said August Arns, 78, outside his polling place in Easton Tuesday. “They think because their name is Kennedy they should be in office.”

But for others, the Kennedy political brand is still the gold standard. Craig Barger, 64, of Easton, said he voted for Kennedy, “first of all, because he’s a Kennedy.”