Elizabeth Warren endorses Texas U.S. Rep. Henry Cuellar's Democratic primary challenger

The Democratic presidential candidate backed Jessica Cisneros, the immigration lawyer running to unseat Cuellar.

Congressional candidate Jessica Cisneros.

Democratic presidential candidate Elizabeth Warren is wading into one of Texas' highest-profile intraparty fights, endorsing Jessica Cisneros, the primary challenger to U.S. Rep. Henry Cuellar, D-Laredo.

"The people of Texas’ 28th district are ready for systematic change and deserve a Democrat that will be on the side of working people; not the side of big money and obstructionist Republicans," Warren, the U.S. senator from Massachusetts, said in a statement Monday morning. "I believe Jessica Cisneros is that fighter."

Cisneros, a young immigration attorney from Laredo, has the backing of Justice Democrats, the progressive group famous for helping elect freshman U.S. Rep. Alexandria Ocasio-Cortez, D-N.Y., last year. Cuellar is among the more conservative Democrats in the House.

"Jessica knows our diversity is our strength and that when progressives are unapologetic about our values and who we’re in this battle for, we win," Warren said. "It’s time Texans had a champion in Congress who does just that."

Warren is by far the most prominent Democrat to back Cisneros yet. Cuellar has brushed off the challenge as out-of-state meddling by Democrats who do not know his South Texas district.

"We told you the outside special interests were coming to take away local jobs," Cuellar tweeted after the endorsement, adding that Warren and Cisneros "share an agenda that would kill" many jobs in the region.

Cuellar campaign spokesman Colin Strother said the Warren endorsement only reinforces the notion that Cisneros "has no support in the district" and is relying on "northeastern extremists."

"I hope more Massachusetts senators endorse her," Strother said.

Warren's endorsement comes as she prepares to head to Texas for the third primary debate, which is Thursday in Houston. She'll first hold a town hall Tuesday in Austin.