Despite speculation that Julian Castro may become Hillary Clinton's running mate, should she win the 2016 Democratic presidential election, the former San Antonio mayor says he has not ruled out running for Texas governor in 2018.

Last week, Castro, the U.S. Secretary of Housing and Urban Development, endorsed Clinton at her rally to kick off her "Latinos for Hillary" campaign in San Antonio. Before the Democratic front-runner took the stage, Castro introduced her as the country's next "Democratic nominee and the next president of the United States." Castro also declared that Clinton "has a strong vision for America's future" and is the candidate best prepared to fix the broken immigration system before a heavily-Latino audience.

When asked if she would consider Castro as a running mate in the 2016 race, the former secretary of state told the U.S. Hispanic Chamber of Commerce that said she would "look really hard at him for anything, because that's how good he is."

However, just in case a Clinton-Castro presidential ticket doesn't come into fruition, the rising Mexican-American leader admitted that he has considered the idea of running for his state's governor.

"Right now, that's not on my radar screen," Castro told KXAN on Sunday at the Texas Tribune Festival in Austin. "Could I say with 100 percent certainty that I would never choose to do that? I can't say that either, but it's not likely to happen."

Castro went on to say that he made the decision to endorse Clinton earlier this year.

"I had told Secretary Clinton a couple of months ago that I would endorse her, so it's been in the works since the summer," he said. "I wanted to do it now, because it's clear the campaign is ramping up, we just had the first Democratic debate, the voting is going to start soon in Iowa and we're running into the holidays."

Castro also said that he is confident Clinton will continue to win over progressive voters.

"I'm confident because of Secretary Clinton's track record of supporting immigration reform, supporting investment in education and supporting expanded health care that has benefited the Latino community that she is going to have very strong support herself," he said.