Rep. Alexandria Ocasio-Cortez (D., N.Y.) has endorsed another liberal challenger looking to unseat an incumbent House Democrat backed by party leadership.

Ocasio-Cortez endorsed progressive Jessica Cisneros in her primary challenge against Rep. Henry Cuellar (D., Tex.), citing Cisneros's status as a "strong new ally in the fight for Medicare for All." Cisneros is supported by Justice Democrats, the same progressive group that backed Ocasio-Cortez and Rep. Ilhan Omar (D., Minn.) in 2018.

Ocasio-Cortez's endorsement marks another act of defiance toward Democratic leadership. House Speaker Nancy Pelosi (D., Calif.) and Democratic Congressional Campaign Committee chairwoman Cheri Bustos (D., Ill.) have endorsed Cuellar.

Ocasio-Cortez attacked Cuellar in her endorsement, accusing the incumbent Democrat of fighting for "big corporate donors" and suggesting he is "quite regressive" compared to his constituency. The Cuellar campaign denounced the freshman congresswoman's endorsement as out-of-state meddling. Ocasio-Cortez, his spokesman said, does not speak for the people of Texas.

"This race is going to be decided by the voters of the 28th District of Texas, not out-of-state PACs and celebrities who seem to be the only people supporting our opponent," said Cuellar spokesman Colin Strother.

Cisneros, 26, did not respond to request for comment. She previously served as an intern in Cuellar's office before accusing him on the campaign trail of being "Trump's favorite Democrat" on the campaign trail. She welcomed Ocasio-Cortez's endorsement on Twitter.

"Our homegrown campaign here in South Texas is making noise across the country — @AOC heard the message of our movement all the way in the Bronx! Proud to have her on our team as we work to fight corporate interests and give power back to the people of TX-28!" she said.

Our homegrown campaign here in South Texas is making noise across the country — @AOC heard the message of our movement all the way in the Bronx! Proud to have her on our team as we work to fight corporate interests and give power back to the people of TX-28! https://t.co/yaXnUWmaEB — Jessica Cisneros for Congress (@JCisnerosTX) October 22, 2019

Ocasio-Cortez's attack on Cuellar puts her at odds with Pelosi, who said she was "proud to support" Cuellar at the Texas Tribune Festival in September. Asked if she would support Cuellar, Pelosi responded "absolutely, absolutely, absolutely, absolutely," adding that she is "very, very proud of Henry's work in the Congress" and "proud to support him—even if I didn't have a policy of endorsing incumbents."

Bustos, who did not respond to request for comment, mirrored Pelosi's comments hours before the festival, rejecting the notion that Cuellar's seat is vulnerable.

"Henry Cuellar knows that district like the back of his hand. I completely support him," Bustos said. "He has very good relationships with the vast majority of his colleagues —who are supportive of him—and I think he'll be fine."

Cuellar slightly out-raised Cisneros last quarter, and currently has a nearly $3 million advantage in cash-on-hand. Cisneros has attacked Cuellar for accepting PAC money, accusing the incumbent congressman of being funded by "out-of-state Republican corporate interests."

"I'm proud to be the only candidate in this primary who is not taking money from out-of-state Republican corporate interests like the Koch Brothers," Cisneros told the Laredo Morning Times. "Our campaign is powered by small-dollar contributors, not corporate PACs and Republicans."

Cuellar has characterized Justice Democrats as a "radical, far-left group," and predicted that Texas voters wouldn't be receptive to its message.

"I’m not going to say anything negative about the young lady. But the group supporting her, the Justice Democrats, it’s a radical, far-left group that wants to impose their vision from New York into South Texas," Cuellar said in June. "And I don’t think the voters are going to be supporting that."

Ocasio-Cortez has butted heads with party leadership in the past. She insinuated that Pelosi was racist during a July spat. She compared Cuellar to former Rep. Joseph Crowley, who was a top House Democrat before he faced Ocasio-Cortez in a 2018 primary. "I now represent one of the most progressive districts in Congress," Ocasio-Cortez told HuffPost. "For decades our representation was quite regressive compared to where my community was at."

Ocasio-Cortez's endorsement of Cisneros reflects continued party division as progressive liberals challenge incumbent Democrats. Though Ocasio-Cortez claims she won't endorse "a challenger to just anyone who disagrees with me," the New York congresswoman has already endorsed Marie Newman, a liberal candidate looking to unseat Rep. Dan Lipinski (D., Ill.), one of the few pro-life Democrats left in the country.