Pelosi’s endorsement could be a significant boon for Kennedy as the race has tightened in recent weeks, with polls showing he and Markey, who served alongside Pelosi in the House for more than two decades, in a near tie headed into the Sept. 1 contest.

Still, Pelosi’s move shocked many on Capitol Hill and puts her at direct odds with her close ally, Senate Minority Leader Chuck Schumer (D-N.Y.), whose Senate campaign arm is designed to protect vulnerable incumbents like Markey. The speaker rarely weighs in on Democratic primary contests, particularly in favor of the challenger.

A Pelosi aide said Kennedy did not ask for the endorsement. But Pelosi felt compelled to weigh in on the race given Kennedy’s efforts in 2018, “which was essential to Democrats' successful efforts winning back the majority," the aide said.

Pelosi was also concerned after the Markey campaign started ramping up its attacks on the Kennedy name, going after “Joe, his family, his supporters and the Kennedy family policy legacy,” the aide added.

“With all due respect, it’s time to start asking what your country can do for you,” Markey said pointedly at the end of one ad, borrowing from the famous quote from Kennedy’s great uncle, President John F. Kennedy.

Kennedy responded with a speech of his own earlier this week flanked by Black community leaders, defending his family and accusing Markey of hampering efforts to further racial equality during his political career.

"We are, once again, at a moment of profound racial reckoning. And we deserve a senator who will not stand by," Kennedy said.

Markey and Pelosi have a long-standing relationship after working decades together during his nearly 40-year tenure in the House. Pelosi even appointed Markey to lead a select House committee on climate change in 2007.

But Kennedy has been loyal to Pelosi since coming to the House in 2013, often working behind the scenes to find avenues for new members to participate in caucus leadership rather than moving to push Pelosi out, like some of his fellow classmates. Kennedy was a significant asset for Pelosi in 2018, using his sway within the caucus to build support for Pelosi’s second tenure as speaker at a time when many others were calling for her to step down.

Markey on Thursday tweeted that Pelosi “is an effective leader who has shattered glass ceilings throughout her career,” adding, “any candidate would be proud to have her endorsement. I congratulate Joe Kennedy on securing her support.”

Pelosi donated $5,000 to Kennedy’s campaign through her super PAC earlier this month. She also gave $5,000 to Markey’s campaign last year.

Pelosi’s endorsement infuriated many progressives, who have faced mounting resistance from the Democratic establishment as they’ve launched their own primary challenges against sitting members.

Rep. Alexandria Ocasio-Cortez (D-N.Y.), who has backed a slew of primary challengers against Democrats since her own insurgent campaign in 2018, said Pelosi’s support of Kennedy over Markey went against the party’s longtime policy against backing challengers, which she said “seems like less a policy and more a cherry-picking activity.”

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

The New York Democrat has been a vocal supporter of Markey — who is the Senate’s lead co-sponsor of her Green New Deal proposal — and has helped to rally her own massive supporter base behind the 74-year-old.

Justice Democrats, which has helped fuel the rise of Ocasio-Cortez and a slew of other progressive challengers, released a blistering statement that accused Pelosi of double standards.

“This move reeks of hypocrisy,” the group wrote in a statement. “The party is setting one standard for progressives and one entirely different standard for the establishment.”

Democratic campaign leaders redoubled efforts in the 2020 election cycle to protect incumbents from threats from the left, attempting to reassure anxious members of the party who watched Ocasio-Cortez topple Joe Crowley, who was considered to be in line to potentially be speaker. The Democratic Congressional Campaign Committee began implementing policies such as a “blacklist" for outside groups that back primary challengers.

House Democrats have argued that their policies to protect incumbents are enforced in their races alone, and don’t carry over to the Senate.

Several other Democratic incumbents — most notably House Foreign Affairs Chair Eliot Engel (D-N.Y.) — have also been ousted by liberal challengers this year. Earlier this month, longtime Rep. Lacy Clay (D-Mo.) also lost a primary challenge, falling to Cori Bush, a Sen. Bernie Sanders-endorsed candidate.