Updated at 11 a.m.: Revised to include a statement from John Cornyn’s campaign.

U.S. Rep. Joaquin Castro has endorsed Cristina Tzintzún Ramirez in the Democratic U.S. Senate race to challenge incumbent Republican John Cornyn.

“Texans deserve better than Sen. John Cornyn, who has spent years fighting against the rights of workers, women, immigrants, and so many others — all while claiming to represent the interests of Texans in DC,” Castro said in a news release. “To replace him, we need someone who understands the Texas of today and who has a positive vision for our future.”

Tzintzún Ramirez co-founded the Workers Defense Project, a group that has fought for the health and rights of construction workers. In her stump speech, she has tried to contrast her activism against Cornyn’s record.

“While John Cornyn was accepting millions of dollars from the construction industry, which here in Texas is the largest employer of undocumented labor, I was making $43,000 a year representing tens of thousands of workers who built our state’s economy and lost their fingers, limbs and lives in the process,” she said at a Plano event.

Cornyn’s campaign said the endorsement was no surprise.

“It’s no surprise one of the most liberal congressmen in Texas is backing the most progressive candidate in this race. Medicare for All, the Green New Deal, and decriminalizing illegal immigration are all hallmarks of the Ramirez-Castro agenda," Cornyn campaign manager John Jackson said in a written statement. "Congressman Castro is desperate to appear relevant after being too scared to run against Sen. Cornyn himself.”

Tzintzún Ramirez is in a tight, 12-person race for Senate. Polls show no candidate has broken from the field. The top contenders include former Air Force helicopter pilot MJ Hegar of Round Rock, state Sen. Royce West of Dallas, former Houston City Council member Amanda Edwards and former U.S. Rep. Chris Bell of Houston.

Last year, Democrats tried to coax Castro or his brother, former Housing Secretary Julián Castro, into the Senate race. Julián Castro was a presidential contender and Joaquin Castro opted to remain in the House.

Rep. Joaquin Castro, D-San Antonio, said: “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." (Andrew Harnik / AP)

“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,” Castro said.

Tzintzún Ramirez is also backed by Castro’s mother, civil rights activist Rosie Castro. The support from the Castro family could help Tzintzún Ramirez win Hispanic voters, who are influential in Democratic Party primaries.

Last year, Tzintzún Ramirez apologized for saying that part of her surname, Tzintzún, is “more Mexican than any Garcia or Lopez,” adding that “we were the only indigenous group in Mexico that were not defeated by the Aztecs. So you know I come from good lineage and I’m ready to defeat John Cornyn.”

The remark caused an uproar on social media after appearing in a profile of her in The Dallas Morning News.

Tzintzún Ramirez is hoping to get the lion’s share of Hispanic votes in the March 3 primary. Early voting began Tuesday.

She called Castro a “tireless advocate for all Texans.”

“I look forward to working together towards a brighter future for Texas,” Tzintzún Ramirez said in a statement.