When Julian Castro, the San Antonio mayor, was announced Tuesday as the keynote speaker at the Democratic National Convention, the most common reaction among Democrats was “Who?”

The first Latino ever tapped for the high-profile primetime address, 37-year-old Castro is the youngest mayor of a top-50 American city. Serving a second term after a 2011 reelection with 82.9 percent of the vote, the Stanford and Harvard Law graduate’s biography lists a number of accolades that make it clear he’s an up-and-comer. He made Time magazine’s list of “40 under 40,” and his life story is the embodiment of what we think of as the striving American. The son of a single mother, Castro and his twin brother, Joaquin, who also has a stellar résumé and is running for Congress in Texas, strike the same chords that Barack Obama did when he was selected to keynote the Democratic convention in 2004—that America is a land of opportunity where merit and hard work are rewarded.

While most national politicos had never heard of him before Tuesday, Castro has been a co-chair of the Obama campaign, and a campaign aide confirms he has been “a very effective surrogate on the stump.” As mayor, he has put into practice some of President Obama’s most cherished ideals by pressing a “new energy economy” and making educational attainment a priority. The Café College he established is a one-stop center to help students prepare for college.

Democrats cheered the choice of Castro—and the Spanish-language network Univision, which broke the news of his selection Tuesday, tweeted that the Castro story gave its website its best traffic day ever.

“He’s young, energetic, and impressive, exactly the kind of message the party wants to send,” says Matt Bennett, a cofounder of the centrist Democratic group Third Way.

“Great pick. Super smart, effective, innovative leader, young and Latino. What’s not to love? The future is now. Another reminder of why conservatives are doing everything they can to suppress the vote,” says John Podesta, founder of the Center for American Progress, a liberal think tank.

Obama campaign spokesman Ben LaBolt calls Castro “a rising leader in the party who has worked tirelessly to build San Antonio’s economy from the middle class out, by making investments in things like clean energy and innovative education programs that will lead to the creation of good-paying, sustainable jobs you can raise a family on.”

Castro will join first lady Michelle Obama on stage Tuesday, Sept. 4, the first night of the convention, to set the tone of a new and diverse electorate but one that also reinforces the traditional values of opportunity and responsibility that underpin the American Dream. “Having both the first lady and Mayor Castro speak on the opening night of our convention will bring together two incredible leaders whose life stories both embody the promise of America—that if you work hard and play by the rules, you can thrive,” said Democratic convention chair and Los Angeles Mayor Antonio Villaraigosa.

The Democratic convention follows that of the Republicans, where Florida Sen. Marco Rubio is expected to play a headlining role, whether or not he is selected as Mitt Romney’s running mate. Rubio and Castro may be opposed ideologically, but the prominence of this new generation of leaders represents a maturing of the Hispanic community as a political force and is a tribute to the parents who sacrificed so their children could get ahead. And that’s the message voters are likely to hear from both of them.