The grandson of assassinated US presidential hopeful Robert Kennedy is launching a bid to enter Congress.

Joseph Kennedy is to battle for the Massachusetts seat now held by retiring congressman Barney Frank.

Kennedy, the son of former US Rep Joseph Kennedy, said he would work hard to earn every vote and if elected would bring the "fight for fairness to the US Congress".

"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," Democrat Mr Kennedy said last night.

Mr Kennedy, 31, recently moved from Cambridge to Brookline, part of the state's newly redrawn 4th Congressional District. The family has deep ties to the Boston suburb.

His grandfather, Senator Robert Kennedy, was shot dead at the age of 42 on June 5 1968 at the Ambassador Hotel in Los Angeles, California, shortly after claiming victory in the state's crucial Democratic primary.

The Kennedy family has seen its influence in Washington fade in recent years as its younger generations have largely shunned public office.

The death of Massachusetts senator Edward Kennedy, Joseph Kennedy's great-uncle, in 2009 left a void for the family. The retirement last year of his son, Rhode Island's US Rep Patrick Kennedy, marked the first time in 63 years that a Kennedy was not serving in elected office in Washington.

Joseph Kennedy, a graduate of Stanford University and Harvard Law School, served in the Peace Corps in the Dominican Republic from 2004 to 2006.

He has not held elective office, but he and his twin brother Matt served as co-chairmen for Edward Kennedy's final re-election campaign in 2006.

Joseph Kennedy has served as a prosecutor for the Cape and Islands district attorney's office and more recently as an assistant district attorney in the Middlesex district attorney's office, a job from which he resigned last month.

Last month he also announced he was engaged to be married to Lauren Birchfield, a fellow Harvard Law School graduate.

Other Democratic candidates in the district, which stretches from the Boston suburbs to Fall River, include Paul Heroux, an Attleboro businessman, and Herb Robinson, a software engineer from Newton.