House Speaker Nancy Pelosi on Thursday endorsed challenger Joe Kennedy over liberal incumbent Ed Markey in the Massachusetts Senate primary, a move that quickly drew a rebuke from progressives like Rep. Alexandria Ocasio-Cortez.

Pelosi threw her clout behind Kennedy, a Democratic member of the House, who is trying to unseat Markey, co-author of the Green New Deal, in the Democratic primary on Sept. 1.

“Never before have the times demanded that we elect courageous leaders as today, which is why I’m pleased to join so many of my colleagues in endorsing Joe Kennedy III for Senate,” Pelosi said in a statement, citing Kennedy's efforts elect more Democrats to win back the House in 2018.

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The statement continued, "He knows that to achieve progressive change, you must be on the front lines leading movements of people. Massachusetts and America need Joe Kennedy’s courage and leadership in the Senate.”

Democratic U.S. Rep. Joseph Kennedy III, D-Mass., announces his candidacy for the Senate on Saturday, Sept. 21, 2019, in Boston. Kennedy will challenge incumbent Sen. Ed Markey in the Democratic primary. (AP Photo/Josh Reynolds)

Progressive immediately slammed Pelosi's decision to work against a sitting liberal incumbent senator, who has been a leader on the Green New Deal and climate change activism -- and has support from Ocasio-Cortez, Sen. Elizabeth Warren and others.

They had balked at the Democratic establishment's efforts to protect incumbents against primary challenges from the left and cried foul Thursday at Pelosi's decision to get involved here.

"This move reeks of hypocrisy: the party is setting one standard for progressives and one entirely different standard for the establishment," Justice Democrats, a pro-AOC group, said in an email to supporters.

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Ocasio-Cortez, a fellow Green New Deal advocate, took to Twitter to express the frustration that progressives are repeatedly slammed for working to primary-challenge sitting members of Congress.

FILE - In this Saturday, Sept. 14, 2019 file photo, U.S. Sen. Edward Markey speaks to delegates during the 2019 Massachusetts Democratic Party Convention, in Springfield, Mass.  (AP Photo/Jessica Hill, File)

"No one gets to complain about primary challenges again," Ocasio-Cortez tweeted, referencing the Democratic Congressional Campaign Committee (DCCC) and its efforts to protect incumbents from primary fights.

Ocasio-Cortez has long railed against a DCCC "blacklist" policy to block vendors from working with progressive challengers in Democratic primaries. And she called on the DCCC again to change its policy that she says prevents political newcomers like herself from running for office.

"It seems like less a policy and more a cherry-picking activity," Ocasio-Cortez said, referencing Pelosi's action and efforts from the D.C. establishment to bankroll a primary challenge to fellow "Squad" member Rep. Ilhan Omar, D-Minn.

Ocasio-Cortez then tweeted a link to Markey's campaign page with the words: "Organize."

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Pelosi isn't endorsing a challenger to a sitting House member, rather a sitting Senate member, which the DCCC doesn't cover. Pelosi endorsed Ocasio-Cortez for her reelection and, like the DCCC, routinely backs House incumbents as part of the overall goal to protect the Democrats' majority.

The Democratic Party's Senate arm, the Democratic Senatorial Campaign Committee, has endorsed Markey for his reelection.

"I'm a big supporter of people in the House," Pelosi told the Washington Post Live Thursday in explaining her endorsement. "You can ask my colleagues, my loyalty is to them."

Pelosi said she got involved in the Senate race because she's very close to the Kennedy family and she wasn't happy with the "assault" on the Kennedys happening in the campaign. Joe Kennedy, 39, is the grandson of Robert F. Kennedy, the former attorney general and senator who was assassinated in 1968 during his presidential campaign.

"Joe didn't ask me to endorse him, but I felt an imperative to do so," Pelosi said.

Kennedy expressed gratitude for Pelosi's support.

“She is a force and a changemaker, and I’m beyond grateful to have her in my corner," he said.