WASHINGTON (AP) — House Speaker Nancy Pelosi endorsed Democratic Rep. Joe Kennedy III on Thursday in his bid to oust Sen. Edward Markey in the Massachusetts Democratic primary, backing a young challenger against a veteran lawmaker with whom she's served in Congress since 1987.

By embracing Kennedy, grandson of the late U.S. attorney general and New York Sen. Robert F. Kennedy, Pelosi sided with a congressional upstart against one of Washington's old guard. Both contenders are among Congress' more liberal Democrats, but Kennedy, 39, has been in the House since only 2013. Markey, 73, has served in Congress since 1975, moving from the House to the Senate in 2013.

"Joe Kennedy represents this party's future. He will help lead Democrats forward on the defining battles of our time," Pelosi said in a fundraising email for him.

The move to spurn Markey, who's been embraced by many progressives, angered Democrats' left flank just hours before the final day of the party's convention, a virtual gathering aimed at rallying the sometimes fractious wings behind presidential nominee Joe Biden.

Pelosi said when Democrats were working to take back control of the House from Republicans during the 2018 elections, Kennedy stumped for candidates across the country.

"From climate change to health care to racial justice, Joe has been a leader in our Caucus organizing us around our core values," Pelosi said in a press release. "We need leaders who are willing to give every inch of themselves to the causes and concerns that unify Democrats. Joe Kennedy represents this Party's future."

A Pelosi aide said she was uncomfortable with remarks Markey had made assailing Kennedy and his family's long political legacy. In one campaign video, Markey paraphrases a famous quote by the late President John F. Kennedy, saying, "We asked what we could do for our country. We went out, we did it. With all due respect, it's time to start asking what your country can do for you."

The aide, who said Joe Kennedy didn't ask for Pelosi's endorsement, was not authorized to publicly discuss the speaker's decision and spoke on condition of anonymity. President Kennedy was Joe Kennedy's great-uncle.

The winner of the hard-fought Sept. 1 primary will be strongly favored to win the November election in the heavily Democratic state.

Pelosi has backed incumbent House Democrats who have faced primary challengers.

Her endorsements this year have included two members of the so-called "squad" of freshman Democratic women of color: Reps. Rashida Tlaib of Michigan and Ilhan Omar of Minnesota. She's also supported centrist Democrats like Rep. Henry Cuellar of Texas, who fended off a progressive challenger in his primary.

But some of party's highest-profile progressives were openly upset with her support for Kennedy.

"This move reeks of hypocrisy," Alexandra Rojas, executive director of the grassroots group Justice Democrats, said in a statement. "The party is setting one standard for progressives and one entirely different standard for the establishment."

Justice Democrats has recruited progressive challengers like Rep. Alexandria Ocasio-Cortez, D-N.Y. Ocasio-Cortez is backing Markey, a fellow author of Green New Deal legislation aimed at curbing climate-altering carbon emissions.

"No one gets to complain about primary challenges again," Ocasio-Cortez tweeted.

The Sunrise Movement, a pro-environment group, called Pelosi's endorsement "an embarrassment" that showed she thinks it's acceptable to support a candidate "who has a rich and wealthy family."

And liberal Rep. Ro Khanna, D-Calif., tweeted that Pelosi's support of Kennedy helps make Markey's case "against dynasty and smoke filled rooms anointing candidates." He predicted the endorsement would backfire and ultimately help Markey.

Kennedy welcomed Pelosi's support.

"She is a force and a changemaker, and I'm beyond grateful to have her in my corner," said Kennedy, the lone member of the political dynasty currently in elected office.

The endorsement follows Kennedy's support of Pelosi during her bid to regain the top position in the House in 2018. Pelosi was nominated as her party's choice for speaker by Kennedy.

Markey called Pelosi a "tremendous, effective leader who has shattered glass ceilings throughout her career."

"Any candidate would be proud to have her endorsement, and I congratulate Congressman Kennedy on securing her support," Markey said in a statement.