San Antonio Mayor Julian Castro, a rising star among Democrats, is poised to make history Tuesday as the first Hispanic to deliver a keynote address at the Democratic National Convention.

The 37-year-old official, who was re-elected in a landslide victory last year, is expected to present himself as a classic American success story -- with a family narrative that begins with his grandmother, a Mexican orphan, who immigrated as a young girl to the U.S. in 1920 and taught herself how to read and write. Castro, the youngest mayor of a big city, has also hinted that his speech Tuesday will be heavy in defense of President Obama's record.

"I’ll be talking about my family’s story," Castro told Fox News. "And that more than anything else, my family’s story, and the story of so many families across the United States from different parts of the country represent an American dream story."

The highly coveted speaking slot will propel Castro into the national spotlight, fueling speculation that the young mayor -- who few had heard of before he was selected for the address -- is eyeing higher office, possibly as governor of Texas or even the country's first Hispanic president. It was just eight years ago when then-Senator Barack Obama used the same primetime speaking slot to rise to national prominence.

The decision to tap Castro as the convention's keynote speaker also speaks to the Democratic Party's push to secure Hispanic voters -- the country's fastest growing minority and a critical voting bloc in the election.

Castro will be introduced at the convention Tuesday night by his closest adviser: his identical twin brother, Joaquin, a Texas state legislator who represents San Antonio and who is favored to win election to Congress in November.

Castro and his brother, who were raised by a single mother, attended Stanford University and then Harvard Law school -- opportunities they attribute in part to affirmative action.

"My brother makes no bones about it. He didn't score 1500 on the SAT," Joaquin Castro told Fox News. "He got a 12-something, so we certainly benefited from it. We still believe that there is a place for it in America."

It was the twins' mother, Rosie, who stressed the importance of education and led them toward a career in politics.

"I have always believed, and I think taught them, that inside everyone there is a leader," Rosie Castro told Fox News.

As a worker's rights activist, she ran for city council at age 23 and drilled her sons on the importance of civic responsibility.

"She would drag us to campaign rallies and she would drag us to organizational meetings and walking neighborhoods," Julian Castro said. "I mean, who wants to do that when they're 10 years old?'" he quipped.

Rosie Castro's lessons, however, eventually took root in her sons, planting the seed for successful political careers.

"I think she instilled in me the sense that I could become a leader -- a confidence in myself that I was someone who had value that I had something to add to the conversation," Julian Castro said.

Fox News' John Roberts contributed to this report.