Editor's note: This story has been updated throughout.
SAN ANTONIO — Accepting Julián Castro's endorsement Thursday, Democratic presidential frontrunner Hillary Clinton addressed speculation he could serve as her running mate, saying she plans to "really look hard at him for anything."
"That's how good he is," Clinton added in a Q&A with Javier Palomarez, the head of the U.S. Hispanic Chamber of Commerce.
Palomarez had started the conversation by prodding Clinton, the former secretary of state, about the vice presidential buzz surrounding Castro, the U.S. housing secretary and former mayor of San Antonio.
“I think really high of him, and I am thrilled to have his endorsement today," Clinton responded, calling Castro and his twin brother, U.S. Rep. Joaquin Castro of San Antonio, two of the "best young leaders in America, regardless of category or the fact that they come from San Antonio."
Julián Castro made his endorsement official while introducing Clinton on Thursday afternoon at a campaign rally in San Antonio. He told the audience that Clinton "knows a little something about our backyard," alluding to her time registering voters during the 197os in South Texas.
"Through the years, she always, always has been there for us, and today we're here for her," Castro said.
He drew a particularly boisterous reaction from the audience when he said he looks forward to watching Fox News call Texas for Clinton on Election Day. She then took the stage, saying Castro's scenario could come true "if everybody here worked to turn Texas blue."
Republicans, who are already treating Castro like Clinton's running mate, found more fodder in his endorsement. Ruth Guerra, a spokeswoman for the Republican National Committee, said in a statement that Castro is "another ethically challenged politician who is more concerned about self-promotion than governing."
Clinton addressed a variety of topics with Palomarez on Thursday, but she was most vocal about what she called the "harsh and inflammatory language" being used by her Republican rivals, particularly on the topic of immigration. Such rhetoric, she said, is "opening the door" for others to not only speak with prejudice but also act on it.
"Somebody needs to say, 'Basta!' Stop it. Enough," Clinton said to applause.