Elizabeth Warren backs Jessica Cisneros, primary challenger of Laredo Democrat Rep. Henry Cuellar

Updated at 11:45 a.m.: Revised to include comment from Rep. Henry Cuellar's campaign.

WASHINGTON — Presidential hopeful Elizabeth Warren is wading deep into intraparty Texas politics ahead of the third round of Democratic debates in Houston, with the Massachusetts senator endorsing the primary challenger of longtime Laredo Rep. Henry Cuellar.

Warren on Monday announced her full-throated support for Jessica Cisneros, a Laredo attorney who is seeking to topple one of the more conservative Democrats left in Congress.

"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 said in a written statement. "I believe Jessica Cisneros is that fighter."

Cuellar campaign spokesman Colin Strother said he wasn't surprised by the endorsement, explaining that both Warren and Cisneros are "socialists" with an "anti-Texas agenda."

"We think senators from Massachusetts will get zero votes in this primary," he said, ripping the duo over policy ideas like the Green New Deal and Medicare for All. "The voters of the 28^th District will get 100% of the votes. So we're going to spend our time trying to convince them."

The endorsement, first reported by the Texas Tribune, adds a high-profile twist as the top Democratic presidential contenders arrive in Texas for the debate on Thursday at Texas Southern University.

Warren, for instance, is scheduled to hold a rally on Tuesday in Austin. Julián Castro, Joe Biden, Kamala Harris and others hoping to take on President Donald Trump are likewise planning fundraisers and other events around the primetime debate.

So while Texas Democratic Party leaders are no doubt relishing the broader spotlight, they may not have anticipated it shining so brightly on a growing intraparty squabble.

The Cuellar-Cisneros clash has already drawn plenty of attention, particularly after the challenger earned the support of Justice Democrats, a progressive group that helped elect well-known freshman Rep. Alexandria Ocasio-Cortez of New York.

Cisneros on Monday touted Warren's support as evidence that "we're building the momentum we need to bring real change to South Texas."

"Our district deserves to be represented by a true Democrat who will work in Washington to improve the lives of her constituents, not corporate donors," the challenger said in a news release, which referred to Cuellar as "Trump's favorite Democrat."

But the longtime Laredo lawmaker has already geared up his campaign, making the case that he knows his district far better than any outsider.

Strother, Cuellar's spokesman, singled out Warren for criticism. He made an apparent reference to the presidential contender's past claims of Native American ancestry by saying that her "past dalliances with the truth have made her a running joke"

He also noted that voters in Cuellar's district overwhelmingly backed Hillary Clinton over Bernie Sanders in the 2016 Democratic presidential primary.

"This district had a legitimate chance to pick a socialist agenda — they rejected it almost 3 to 1," Strother said. "We don't really see this district breaking for a couple of bad Bernie knockoffs when they could've had the real thing in 2016."