Congressman Joe Kennedy, long seen as a rising star, crashed to defeat Tuesday in his bid for a US Senate seat, the first time the storied American political dynasty lost a state election in Massachusetts.

Incumbent Senator Ed Markey, a political veteran who re-cast himself as the fiery liberal in the race, harnessed the state's progressive energy to handily turn back a primary challenge from Joseph Kennedy III, the grandnephew of assassinated president John F Kennedy.

The 39-year-old Kennedy said he called Markey "to congratulate him and to pledge my support" in the months ahead."

Since he is not allowed to be on the ballot as a candidate for both the House and Senate, Kennedy is not running for re-election to his House seat. Come January, there will be no one from the Kennedy clan in elected office.

The grandson of slain attorney general Robert F Kennedy is part of the family's fourth generation to carry the political torch, entering the House of Representatives in 2013.

"Every race I've ever run, I've made it clear it's me on the ballot," Kennedy told AFP as he stumped for 11th-hour votes Tuesday in Boston.

For the Kennedy clan, there was much on the line. For all but two years since 1947 -- nearly a third of the history of the United States -- a Kennedy has served in elected office.

They are the quintessential American dynasty, one of the closest things the country has to royalty.

Like his presidential great-uncle, the lanky, red-haired Kennedy ran on the promise of a new generation.

And he had received the rare endorsement of House Speaker Nancy Pelosi, who in 2018 appointed him to deliver the Democratic response to President Donald Trump's State of the Union speech.