U.S. Rep. Joaquin Castro, D-San Antonio, is endorsing Cristina Tzintzún Ramirez in the crowded Democratic race to challenge U.S. Sen. John Cornyn, R-Texas, providing one of the biggest endorsements yet in the primary.

"To replace [Cornyn], we need someone who understands the Texas of today and who has a positive vision for our future," Castro said in a statement. "I have a tremendous amount of respect for each of our Democratic candidates in this race, but with her extensive track-record working on behalf of Texans to secure living wages, protect immigrant communities, and engage our young and diverse electorate, I'm proud to support Cristina Tzintzún Ramirez."

Tzintzún Ramirez is a progressive organizer who co-founded the Workers Defense Project and founded Jolt, which seeks to mobilize Latino youth.

Castro's endorsement comes the same day early voting begins for the primary, which features 12 candidates and is all but guaranteed to go to a runoff. A University of Texas/Texas Tribune Poll released Monday suggested that MJ Hegar is the favorite to advance to an overtime round, but the No. 2 spot is up for grabs.

Castro, chairman of the Congressional Hispanic Caucus, is the most prominent Texas Democrat yet to pick sides in the primary, and his endorsement may be the most significant besides that of the Democratic Senatorial Campaign Committee, which backed Hegar in December. Another candidate, Dallas state Sen. Royce West, has the most congressional endorsements in the primary. Those four endorsements come from Reps. Sylvia Garcia of Houston, Eddie Bernice Johnson of Dallas, Al Green of Houston and Marc Veasey of Fort Worth.

Castro considered running against Cornyn last year but opted against it. Cornyn's campaign taunted Castro over that decision in responding to his endorsement of Tzintzún Ramirez.

"It's no surprise one of the most liberal congressmen in Texas is backing the most progressive candidate in this race," Cornyn campaign manager John Jackson said in a statement. "Congressman Castro is desperate to appear relevant after being too scared to run against Senator Cornyn himself."

Last week, Castro's mother, the civil rights activist Rosie Castro, endorsed Tzintzún Ramirez. His brother, the former presidential candidate Julián Castro, held open the possibility he, too, could endorse in the primary during a Thursday interview with The Texas Tribune.

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