(Reuters) - U.S. Representative Joseph Kennedy III, a scion of the famed political dynasty, on Saturday launched a 2020 primary challenge against fellow Democrat Edward Markey for his U.S. Senate seat from Massachusetts.

Kennedy’s decision to seek the Democratic nomination to one of the state’s two U.S. Senate seats set up what will be a closely watched battle between the 38-year-old grandson of the late Senator Robert F. Kennedy and the 73-year-old Markey, the state’s longest-serving member of Congress.

Robert F. Kennedy, who also served as U.S. attorney general, was assassinated in June 1968 as he campaigned for president.

“I’m running for U.S. Senate,” Joseph Kennedy III said in a post on his Facebook and Twitter pages. “This isn’t a time for waiting, for sitting on the sidelines, or for playing by rules that don’t work anymore. This is the fight of our lives, the fight of my generation — and I’m all in.”

The announcement was posted minutes before he kicked off his campaign with a speech in East Boston, surrounded by family members and local supporters.

He recounted his family’s long ties to the neighborhood, noting that an ancestor settled there in 1848 and that his grand-uncle, President John F. Kennedy, and father, former Representative Joseph Kennedy II, represented it in Congress.

He did not mention Markey, instead launching a broadside against President Donald Trump and the Senate’s Republican majority, accusing them of policies that have hurt working Americans and the environment.

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Joseph Kennedy III and Markey both belong to their party’s progressive wing. But Kennedy brings his family’s star power to next year’s race, potentially giving Markey one of the toughest contests of his political career.

Markey was first elected to the House of Representatives in 1976. He won a 2013 special election to a Senate vacancy and the following year comfortably won re-election to the seat.

During his House career, Markey was one of a handful of lawmakers who led the fight for legislation, which narrowly passed the House in 2009, to bring down carbon emissions that contribute to climate change.

Markey has earned the endorsement of many of Massachusetts’ top Democrats. They include the senior U.S. senator, Elizabeth Warren, a leading contender to be the Democratic Party’s nominee to run against Trump next year.

Warren endorsed Markey despite a long friendship with Joseph Kennedy III, who was one of her former law school students and helped launch her presidential campaign.

News reports say that early polling indicated that Joseph Kennedy III started the race as the front-runner.

Massachusetts is a strong Democratic state and the winner of the party primary next year is likely to win in the Nov. 3, 2020, general election.