Backed by the same progressive group that helped elect Rep. Alexandria Ocasio-Cortez, immigration lawyer Jessica Cisneros announced Thursday her intent to unseat longtime Rep. Henry Cuellar, D-Texas.

The contest is likely to become a key flashpoint in the larger national struggle between the insurgent left and the Democratic Party establishment, a fight that exploded into the open when Ocasio-Cortez, now a freshman congresswoman from New York, toppled a top-ranking House Democrat in a 2018 primary.

Cuellar has long been on the left's target list because he represents a safe Democratic district, but is not as progressive as liberals would like on issues such as guns, abortion and immigration. According to a CQ Roll Call analysis, he voted with President Donald Trump more than any Democrat but one in the previous Congress, at 67 percent of the time. He's also one of the only House Democrats to receive the backing of the National Rifle Association in his campaign.

“Here’s the truth: Henry Cuellar fights to protect Trump and the big corporations,” Cisneros said in a video announcing her primary campaign. "I'm fighting to end the separation of families. I’ll fight to pass a $15 minimum wage, Medicare For All and the Green New Deal, so that we can create jobs here at home.”

Justice Democrats, a political action committee that backed Ocasio-Cortez's primary bid, announced its support for Cisneros on Thursday, the group's first endorsement of the 2020 cycle.

"She represents the voices we so desperately need in Congress right now — millennial, working-class, Latina, first-generation immigrant, and dedicating her life to giving back to her community," the group’s executive director, Alexandra Rojas, said in a statement.

Cuellar, who is serving his eighth term after being elected in 2004, has argued his district is misunderstood and is more conservative than it appears, despite its blue partisan lean.

"We feel very strongly that the Congressman represents the values of his district very well and that he knows and understands the priorities for his constituents, and we look forward to comparing his record of service to any candidate that gets in the race," Colin Strother, Cuellar's campaign spokesman, told NBC News in a phone interview.

In the 2018 midterm elections, progressive insurgents had better luck in deep blue districts like Cuellar’s than purple battleground ones, where most of their favored candidates lost to Republicans.

Progressive and women’s groups are already backing a challenge to Rep. Dan Lipinski, D-Ill., a moderate who opposes abortion rights, in a Democratic-leaning Chicago district. And more primary battles are expected to come.

However, the Democratic Congressional Campaign Committee, whose job it is to defend and elect Democratic members of Congress, has doubled down on its pledge to support incumbents, threatening to blacklist consultants and vendors who work for primary challengers to sitting Democrats.

Progressives want to grow the ranks of allies in Congress and build a bulwark that can drag the party further to the left in policy debates.

"That means in 2020 running primary challenges in safe Democratic seats with a slate of young, diverse progressive candidates," Data for Progress co-founder Sean McElwee wrote in an essay for In These Times, a progressive monthly magazine.

Strother, Cuellar's spokesman, accused Justice Democrats of imposing their model on a district where their views were out of step.

"Justice Democrats is a for-profit New York organization that thinks they know this district better than the lifelong resident Congressman," he said. "We don't think that the border and The Bronx are the same politically and we feel confident that the Congressman will be re-elected to another term."