Sen. Bernie Sanders, I-Vt. on Monday joined Rep. Alexandria Ocasio-Cortez, D-N.Y., in backing the progressive challenge by Jessica Cisneros against longtime moderate Democratic Rep. Henry Cuellar of Texas.

Cuellar, a key centrist negotiator in the House of Representatives, is being targeted in part because of his resistance last year to linking President Biden’s massive social infrastructure and climate change spending bill to the bipartisan infrastructure package that was passed into law. And Cuellar, who’s currently under investigation by the FBI for alleged improper ties to Azerbaijan, is also under attack from the left for his ties to and support from the oil industry and for his repeated votes against abortion rights.

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The 28-year-old Cisneros came very close to ousting Cuellar in the 2020 Democratic primary in Texas's 28th Congressional District, a once overwhelmingly blue district that has become more competitive. The district is located the southern part of the Lone Star State and includes the outskirts of San Antonio.

Sanders, who’s backing Cisneros for a second straight election cycle, said in a statement that "Jessica knows that real change comes from the bottom on up, not the top on down." The longtime Vermont senator, progressive champion and runner-up in the 2016 and 2020 Democratic presidential nomination races, emphasized that Cisneros "will fight for the working class in Congress and together we will build a movement to transform this nation so that it works for all our people."

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Cisneros, in a statement thanking Sanders for his endorsement, took aim at Cuellar, arguing that "we're taking on Cuellar's Republican corporate backers with a positive vision for South Texas which includes delivering on Medicare for All, reproductive freedom, and good-paying union jobs."

Cisneros is also being backed by the Justice Democrats, the progressive group founded in 2017 that recruited Ocasio-Cortez, who stunned the political world with her 2018 Democratic primary defeat of longtime Rep. Joe Crowley of New York. The same year Justice Democrats also backed fellow "Squad" member Rep. Ayanna Pressley of Massachusetts, in her primary ouster of veteran Rep. Mike Capuano in the Democratic primary. They also supported fellow Squad members Reps. Ilhan Omar of Minnesota and Rashida Tlaib of Michigan in their 2018 elections to Congress, and two years later backed Reps. Cori Bush of Missouri and Jamaal Bowman of New York as they defeated longtime House Democrats in the 2020 primaries.

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Ocasio-Cortez, who endorsed Cisneros last month, headlined a rally on Saturday in San Antonio for her and Greg Casar, another Democratic congressional candidate backed by the Justice Democrats.

Henry Cuellar

Rep. Henry Cuellar (D-TX) speaks on southern border security and illegal immigration, during a news conference at the U.S. Capitol on July 30, 2021 in Washington, DC.  (Photo by Kevin Dietsch/Getty Images) (Getty Images)

Cuellar, who has become an increasingly vocal critic of progressives the past couple of years, has accused the party’s left flank of alienating voters in South Texas, where the GOP has made gains in recent elections.

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A week ago, ahead of the Ocasio-Cortez-headlined rally, the congressman highlighted that "the voters will decide this election, not far-left celebrities who stand for defunding the police, open borders, eliminating oil & gas jobs, and raising taxes on hard working Texans."

Monday marked the beginning of a week and a half of early voting ahead of the March 1 primary in Texas.