Massachusetts Sen. Ed Markey held onto his Senate seat, defeating a rising star from the Kennedy dynasty in the deep blue state’s Democratic primary Tuesday.

Rep. Joseph Patrick Kennedy III, who currently represents the state’s 4th Congressional District, called Mr. Markey to concede the race about two hours after polls closed, according to multiple reports.

It marked the first time in history that a member of the Kennedy family lost an election in the Bay State. Mr. Kennedy, 39, is the grand-nephew of President John F. Kennedy and the late Sen. Ted Kennedy. His father is former Rep. Joseph P. Kennedy II.

Mr. Markey, 74, enjoyed endorsements from the progressive wing of the Democratic Party including his fellow state Sen. Elizabeth Warren and millennial democratic socialist Rep. Alexandria Ocasio Cortez of New York.

Though he’s been in Washington since 1976, serving in the House of Representatives for 44 years before entering the Senate in 2013, Mr. Markey ran as the “outsider” candidate.

Recent surveys prior to Tuesday’s primary election showed the incumbent with an advantage.

In two of them, Mr. Markey had a double-digit advantage well outside of the survey’s margin of error. A University of Massachusetts at Amherst poll had Mr. Markey showing a surge of support. About 50% said they would support him if the election were held now, whereas only 32% said the same for Mr. Kennedy. In February, the poll showed the two candidates nearly tied.

Mr. Kennedy’s name and his endorsement from party leaders like House Speaker Nancy Pelosi, California Democrat, made him viewed by voters as an establishment favorite.