The two Massachusetts men are competing for a seat in the Senate. Pelosi decided to throw her weight behind Kennedy, even though she has previously argued that all incumbents need to be supported.
In a statement, Alexandra Rojas, the executive director of the progressive group Justice Democrats, accused Pelosi of hypocrisy.
"This move reeks of hypocrisy: the party is setting one standard for progressives and one entirely different standard for the establishment."The Sunrise Movement, a climate change advocacy organization, described Pelosi's endorsement as "embarrassing."
"We're honestly shocked she had the gall to invoke social movements in endorsing Joe Kennedy. This endorsement is embarrassing because it plainly reveals a ridiculous double standard," the group said.
A spokesperson for the Bernie Sanders-affiliated Our Revolution echoed these sentiments, saying that if the party establishment "think we are just going to sit on our hands and take it, they have another thing coming."
California Rep. Ro Khanna, who has endorsed Markey, predicted that Pelosi's support for Kennedy will backfire.
"Pelosi endorsing Kennedy is just evidence that supports Ed Markey case against dynasty and smoke filled rooms anointing candidates. Bold prediction: it will backfire," he tweeted.
"No one gets to complain about primary challenges again," New York Rep. Alexandria Ocasio-Cortez wrote in a Twitter message, pointing out that the Democratic Congressional Campaign Committee has blacklisted political consulting firms that work with primary challengers.
Ocasio-Cortez also accused Pelosi and other leaders of leaving Rep. Ilhan Omar of Minnesota to fend for herself, even though she faced a formidable challenger earlier this year.
Omar's challenger, Antone Melton-Meaux, was endorsed by several notable Democrats and raised $3.2 million.
By throwing her support behind Kennedy, whom she described as the future of the Democratic Party, Pelosi broke with a number of prominent Democrats, including Senate Minority Leader Chuck Schumer.
According to Politico, Pelosi's endorsement could be significant for Kennedy, given that he and Markey, who has been in Congress for nearly four decades, are polling neck-and-neck with less than two weeks until the election.As the publication noted, in the 2020 election cycle, Democratic leaders strongly pushed back against primary challengers from the left, doing all in their power to protect moderate incumbents.
Still, progressives racked up some major victories this year.
Longtime Rep. Lacy Clay of Missouri was defeated by insurgent candidate Cori Bush and Rep. Eliot Engel lost his seat in the House to progressive Jamaal Bowman.