New York Congresswoman Alexandria Ocasio-Cortez has endorsed Jessica Cisneros, an immigration and human rights attorney seeking to unseat Texas Congressman Henry Cuellar in a Democratic primary.

Like Ocasio-Cortez in her 2018 campaign to unseat New York Democratic Congressman Joe Crowley, Cisneros is challenging Cuellar from the left and, at 26 years old, is hoping to replace Ocasio-Cortez as the youngest woman in Congress.

"Last year, NY-14 elected me as the youngest woman to serve in Congress. Let's help Jessica Cisneros shatter that record in 2020," Ocasio-Cortez wrote in a tweet on Tuesday, adding that Cisneros is a "powerful, progressive fighter."

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Ocasio-Cortez's endorsement is the first Cisneros has received from a sitting House member; she already has the backing of Senator Elizabeth Warren, a prominent progressive voice and leading presidential candidate. Cisneros also has the backing of Justice Democrats, the advocacy group that backed Ocasio-Cortez in her primary campaign, and EMILY's List, a politically powerful organization which aims to elect pro-abortion rights Democratic women.

Cisneros has criticized Cuellar as "Trump's favorite Democrat." Cuellar was one of only three House Democrats in 2018 to be supported by the National Rifle Association. He also has supported Republican-led immigration bills, including one that would crack down on so-called sanctuary cities.

According to FiveThirtyEight, Cuellar has voted in line with President Trump 49% of the time since Mr. Trump entered office, even though Hillary Clinton defeated Mr. Trump by nearly 20 percentage points in Cuellar's expansive Southern Texas district in 2016.

Cuellar has a significant cash advantage, as he raised over $375,000 in the third quarter and has over $3 million in cash on hand. Cisneros raised over $317,000 in the third quarter, and has almost $300,000 in cash on hand. However, Cisneros had more contributions that came from individual donors, rather than political action committees.

Ocasio-Cortez's endorsement is particularly influential among progressive voters, as evidenced by her endorsement of presidential candidate Senator Bernie Sanders on Saturday.

However, Cuellar's camp waved off Ocasio-Cortez's endorsement of Cisneros. Colin Strother, a spokesperson for Cuellar's campaign, said in a statement to CBS News that they were "focused on voters in the district."

"This race will be decided by voters from the border, not the Bronx. We are focused on voters in the district, not celebrity endorsements and out of state PACs like our opponent," Strother said.