When Castro broke through the laughter to identify himself, Obama replied: "I know who you are." He sat in the guest box of First Lady Michelle Obama at last year's State of the Union and he has been to the White House more times (12) than the chairman of the Joint Chiefs of Staff (11), according to National Review.
For Democrats, Castro's promise is about cultivating a figure from their party who can unlock the potential of the Latino vote. While a record 12 million Latinos are expected to vote in 2012, 10 million more are eligible but don't vote. Democrats believe those potential voters could make red states like Arizona and Texas perennially competitive. And perhaps a figure like Castro – young and Mexican-American like the bulk of the Latino population in the U.S. – can make that happen.
"You're not considered one of the battleground states, although that's going to be changing soon," President Obama told a crowd of donors at a fundraiser in San Antonio, where Castro was in attendance.
But Castro says that there are significant problems in motivating Latinos to vote in areas where they don't. The population is overwhelmingly young and is not as educated as the general population, two factors which contribute to lower voting levels.
"I don't think there is any easy answer." he said. "It will also take getting out there to the grassroots and convincing them of why they should be participants in the process. And there hasn't been enough of that, especially in a state like Texas where it's written off as being controlled by one party."
There's no question that the choice of Castro is also meant to pay short-term dividends for Obama, who needs a strong showing from Latino voters in November to win a second-term. Though Castro's home state is not in play for Democrats, his Mexican-American heritage fits that of most Latino voters in western battleground states like Nevada and Colorado.
"I think I was chosen because it's one more signifier of how important the Latino community is to President Obama," says Castro. "It's one more reminder that for Latinos, he's been a very effective advocate over these last few years."
Castro also has his doubters, whose criticisms echo those of then-candidate Barack Obama. They say his record is too thin to be considered for national office. For example, the Milken Institute last year named San Antonio as the top job-creating city in the U.S. but conservatives say that's because of Texas' low-tax policies and not Castro's efforts. They also note San Antonio continues to suffer from large high school drop-out rates despite Castro's initiatives to improve education.
Some even scoff at Castro's 81 percent support in his reelection race last year, saying that only 7 percent of city voters showed up in the off-year election.
"We don't need a Spanish-speaking Obama," said George Rodriguez, president of the conservative South Texas Political Alliance. "We don't need affirmative action politicians. By that I mean, we need the best-qualified person as well as a person who has the values of America at heart to lead us."
While Democrats have heaped sky-high expectations upon him, Castro is working to downplay them ahead of his keynote speech.