“As an assistant district attorney, I fought for fairness every day in the courtroom,” Kennedy said. “I volunteered in a legal aid clinic when I was a law student, helping tenants who were mistreated by landlords and banks.”

Kennedy also gave a nod to his family legacy in announcing his run for the congressional seat being vacated by US Representative Barney Frank, a Democrat, after 32 years on Capitol Hill.

“My family has had the great privilege of serving Massachusetts before,” he said. “They taught me that public service is an honor, given in trust, and that trust must be earned each and every day. That’s exactly what I intend to do.”

The Democrat also plans to visit five locations across the Fourth Congressional District today, starting at 7:45 a.m. at the Newton Center MBTA stop.

“I believe this country was founded on a simple idea: that every person deserves to be treated fairly, by each other and by their government, but that’s not happening in America anymore,” Kennedy said in a statement on Wednesday night.

“I’ve spoken to people from across the Fourth Congressional District — from Newton to Fall River — who believe that Washington no longer works for them. I will work hard to earn every vote and if elected bring that fight for fairness to the US Congress,” he added.

Three lesser-known Democrats, Herb Robinson, Paul Heroux, and Jules Levine, are also competing for their party’s nomination, as are at least two Republicans, Sean Bielat, a former Marine who challenged Frank for reelection in 2010, and Elizabeth Childs, a Newton psychiatrist.

The announcement is perhaps the least surprising in recent Massachusetts political history, coming a month-and-a-half after Kennedy announced he was forming a congressional exploratory committee.

He subsequently quit his job as an assistant Middlesex district attorney, moved to Brookline from his mother’s home in Cambridge, arranged a series of fund-raisers in Washington next week, and received several high-profile labor endorsements from the AFL-CIO and Service Employees International Union.

Today’s events codify his desire to be the first member of his famed political family to return to Washington since his cousin, former US Representative Patrick Kennedy of Rhode Island, ended his own service in the US House last year.

His grandfather and his late uncle, Edward M. Kennedy, both served in the Senate, while his uncle John F. Kennedy served in the US House and US Senate before being elected president in 1960.

His fund-raisers next Wednesday will take place on what would have been Edward Kennedy’s 80th birthday.

A recent poll showed the younger Kennedy with a wide lead over Bielat in a hypothetical matchup, largely attributable to the Kennedy family’s name recognition.

Nonetheless, Kennedy has declared that he will not rely on family recognition but on shoe-leather campaigning to win the race.

Kennedy, 31, is one of twin boys born to the former congressman and his first wife, Sheila Rauch.

Kennedy is not known for having his father’s fiery demeanor, as much as his mother’s congenial disposition, and in his early statements, he has channeled the underdog passion that was a hallmark of his late grandfather.

His strategy of formally announcing his candidacy by video, and then following it with a tour of Massachusetts communities, mimics the process used last summer by fellow Democrat Elizabeth Warren as she kicked off her US Senate campaign.

Both Kennedy and Warren are served by the same political and media consultants, Doug Rubin and Kyle Sullivan.

The video format allows Kennedy, a political newcomer, to deliver a polished speech without the imposing presence of an audience. It also offers the opportunity to include testimonials by supporters, as well as family photos.

Touring various communities, meanwhile, will earn him a bounty of free media coverage.