Former Marine and technology executive Sean Bielat and former Middlesex County assistant district attorney Joseph P. Kennedy III will face off for the chance to represent Massachusetts’ Fourth Congressional District and fill the seat held for decades by retiring Congressman Barney Frank ’61-’62.

Bielat and Kennedy each won their respective primaries handily, sweeping past their party opponents by approximately 70 and 90 percent respectively. Both Bielat and Kennedy hold Harvard degrees, the former from the Kennedy School of Government and the latter from the Law School.

In the Fifth Congressional District—which includes Harvard’s Cambridge campus—consulting actuary Tom Tierney edged out lawyer Frank J. Addivinola Jr. and financial consultant Jeffrey M. Semon for the Republican nomination and the chance to take on Edward J. Markey, the longest-serving Congressman in the Massachusetts delegation.

Democrats are expected to be the frontrunners in both congressional races. Kennedy, a 31-year-old first time candidate, has caught national attention since he declared his candidacy in January, attracting big money and political support in and out of state. Markey has won 18 consecutive elections with 60 percent of the vote or more.

“I got into this race because I believe this country was built on a simple promise: that each of us deserves a fair shot,” Kennedy wrote in a statement Thursday night. “I promise that no one will work harder than our campaign to earn every vote. I look forward to having a real conversation over the next two months about the issues that matter to the people of the 4th.”

Both Kennedy and Markey will be running in traditionally Democratic districts. Though those districts were redrawn earlier this year after the 2010 Census stripped Mass. of one of its Congressional seats, experts expect the Fourth and Fifth will remain strongly Democratic.

Bielat beat out his closest rival, former Massachusetts Department of Mental Health commissioner Elizabeth Childs, with roughly 70 percent of the vote. Though he was the Republican favorite heading into Thursday's primary, Child’s last minute media push and a unfolding controversy surrounding a Federal Election Commission fine leveled against Bielat had rendered the outcome less certain in recent days. Ultimately, these issues were not enough to tighten the race.

“We need to make some tough decisions about the direction that our country is headed,” Bielat said in a statement Thursday night. “Do you trust a recent college graduate with three years of experience or do you trust a Marine with an MBA and extensive experience in national security and business?”

Bielat’s general election contest with Kennedy will not be his first in the Fourth District. In 2010, Bielat fell 11 points short of unseating Frank, closer than any Republican challenger had come in decades. Frank’s retirement announcement earlier this year opened the field for Kennedy and Bielat to enter the race.

Markey is expected to easily best Tierney, a perennial candidate, in the general election. In either case, a new Congressman will represent a large part of Cambridge. That area, previously represented by Congressman Michael Capuano as part of the 8th Congressional District, was redistricted earlier this year into a new, Fifth District. Capuano, who was unopposed Thursday, will now represent the rest of Cambridge in a new 7th District.

Locally, Cambridge City Councillor Marjorie C. Decker won 25th Middlesex District Democratic primary with 84 percent of the vote. The house district is the only one entirely in Cambridge and encompasses Harvard’s campus. With her primary victory Thursday and without a Republican opponent in November, Decker is only a formality away from a seat on Beacon Hill.

Both candidates for Massachusetts junior U.S. Senate seat—Harvard Law School professor Elizabeth Warren and incumbent U.S. Senator Scott Brown—were uncontested in Thursday’s primary. Fresh off a nationally-televised speech at the Democratic National Convention Wednesday night, Warren’s only public appearance of the day came en route to cast her own ballot in Cambridge. Brown was equally low-key, accepting the endorsement of a former Springfield mayor.

As predicted, without any contested statewide primary races, few voters turned out for the rare Thursday election.

—Staff writer Nicholas P. Fandos can be reached at nicholasfandos@college.harvard.edu.