The Kennedys are officially back into the family business.

Joseph Kennedy III, a scion of the famous political family, announced Thursday that will run for the Massachusetts House seat being vacated by Rep. Barney Frank (D).

“Congressman Frank has done so much for Massachusetts and America during his service in Congress and he leaves very big shoes to fill,” Kennedy said in his announcement video for the 4th district. “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.”

He promises to fight for a “a fair job plan,” a better educational system, “a fair tax code” and “fair housing policy.”

The grandson of Robert F. Kennedy and son of former Rep. Joseph Kennedy II, the 31-year-old Kennedy was working as a prosecutor and before that as an assistant district attorney in Massachusetts. He also seerved as a Peace Corps volunteer. His twin brother Matt worked on the late Sen. Ted Kennedy’s last reelection campaign.

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Should he win, he will be the only Kennedy in Congress — a decades-long line that was broken when Sen. Ted Kennedy (D-Mass.) died in 2009 and Rep. Patrick Kennedy (D-R.I.) retired in 2010. (Another Kennedy in Congress?)

Kennedy’s father struggled in the House, picking fights with colleagues and driving away staff.

Kennedy is not the only candidate in the Democratic primary, although his pedigree makes him an early favorite and he is already winning union support. Businessman Paul Heroux, software engineer Herb Robinson, and sociologist Jack Porter are also running.

View Photo Gallery: A number of members of the famous Kennedy family have gone on to a career in politics.

Frank said Thursday on CSPAN that he thinks Kennedy will be a worthy successor.

“I am very impressed with him,” Frank said. “I’m very enthused about Joe. I think he’s going to win.”

Frank is retiring rather than facing new voters in a district that is changing dramatically thanks to redistricting, although it remains overwhelmingly Democratic.

Republicans were quick to paint Kennedy as a stand-in for President Obama.

“What Joe Kennedy lacks in experience, he makes up for in his commitment to pursue the same job-destroying policies that have already devastated families in Massachusetts,” National Republican Congressional Committee Spokesman Nat Sillin said in a statement.

An early UMass Lowell/Boston Herald poll found Kennedy would beat Republican Sean Bielat, who ran against Frank in 2010, 60 percent to 28 percent.

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