Julián Castro endorses Jessica Cisneros, the candidate challenging U.S. Rep. Henry Cuellar

Julián Castro is endorsing Jessica Cisneros, the primary challenger to U.S. Rep. Henry Cuellar of Laredo — a fellow Texas Democrat who serves with Castro's brother in Congress.

"The people of South Texas deserve a homegrown champion for hardworking families, good paying jobs, better health care, and immigrants’ rights," Castro said in a statement first shared Friday with The Texas Tribune. "I’m proud to endorse Jessica Cisneros because she will put the people of South Texas first, not special interests or Washington politics."

Castro — the former presidential candidate, U.S. housing secretary and San Antonio mayor — is Cisneros' biggest in-state endorser yet after months of nods from national progressive figures and groups. One of her earliest big-name backers was U.S. Sen. Elizabeth Warren, the White House contender whom Castro backed last month after ending his own run.

Castro's twin brother is U.S. Rep. Joaquin Castro of San Antonio, the chairman of the Congressional Hispanic Caucus, which includes Cuellar.

Cuellar has framed the primary as a battle against out-of-district forces, touting a list of over 100 local endorsements. The centrist lawmaker has also drawn support from unconventional corners for a Democratic primary, including earlier Friday from the LIBRE Initiative Action — marking the first time the Koch political network has endorsed a federal Democrat.

"Julián’s my friend and a former client, but anyone that knows him knows that he has an unparalleled streak of backing losers," Cuellar spokesman Colin Strother said. "I can hardly recall an endorsement that he made that worked out. I hope his streak continues."