Keynote Speaker San Antonio Mayor Julian Castro Will 'Speak from the Heart'

San Antonio Mayor Julian Castro says he will "speak from the heart."

Sept. 4, 2012 -- When San Antonio Mayor Julian Castro speaks tonight -- at times in Spanish -- during primetime at the Democratic National Convention, many people will think of another young, energetic graduate of Harvard Law school who was propelled to prominence by his keynote speech eight years ago.

"A lot of folks have asked about President Obama's 2004 speech. He's a person of unique ability and talent," Castro, 37, told ABC News anchor Diane Sawyer. "I'm going to be myself and I think if I do a good job, and I'm myself and I speak from the heart, it'll resonate with folks."

As the youngest mayor of any top 50 U.S city and the first Hispanic to deliver a keynote address at a Democratic convention, both Democrats and Republicans are already eyeing Castro as a potential presidential candidate down the road. Former George W. Bush adviser Mark McKinnon has been quoted as saying that Castro "has a very good chance of becoming the first Hispanic president of the United States."

Castro insists that he has no presidential aspirations.

"I don't have a passion for that ... to try and become president. That's not what I've woken up and said every morning, 'Hey, I think I can do this or I want to do that,'" Castro said. "The reason I got into politics in the first place was that I had a very competitive streak for my city and that I wanted to make it the greatest city in the U.S., and we've made great strides."

Castro said he is a political realist.

"I'm also in Texas. Texas has 29 statewide offices, and the count between Republicans to Democrats is 29 to zero, so I'm not a fool," he said.

Tonight, Castro says he wants people to come away from his speech focused on one word, "opportunity." He believes it is not only the key to helping Obama get reelected, but also the key to America's success in the future.

"It's not going to be the personalities that matter, it's going to be the policies," he told Sawyer. "In San Antonio, we're convinced that brain power is the new currency of success in the 21st century global economy. It's those young people who care most about that, they want to graduate from high school, graduate from college, or go into the military, prepare themselves with the skills it will take to compete in that global economy, and there's only one candidate who is making the investments they need to do that, and that's President Obama."

Castro said that part of the address will be in Spanish, and the Obama campaign is hoping Castro will help them in the battle for Hispanic votes. Despite his Mexican roots, Castro conceded that his "English is a lot better" than his Spanish, because his mother, Rosie, wanted him and his identical twin brother, Joaquin, to speak and think in English.

Joaquin Castro will introduce his brother this evening. The two have been virtually inseparable since entering the world together in 1974.

"I can't get rid of him ... [he] is also in politics, and we've grown up together, went to college together, law school together," Castro said. "My brother has been the dreamer between the two of us. So it's just very meaningful for me to get to share the moment with him, like we've shared everything in our lives together."