SAN ANTONIO — San Antonio Mayor Julian Castro is a rising star in the Democrats’ firmament, with a collection of mostly favorable media profiles, a landslide re-election last year and speculation about whether he will become the governor of Texas or even the country’s first Hispanic president.

The 37-year-old twin had a chance to shine in prime time Tuesday as the first Latino to deliver the keynote address at his party’s national convention.

What hints Julian Castro has dropped about his speech suggest a script heavy on a defense of President Barack Obama’s record, along with a telling of the story of how he and his identical twin brother, Joaquin, grew up, raised by a single mother.

Joaquin, a Texas state legislator now representing San Antonio and poised to win election to Congress in November, introduced his brother at the convention opener Tuesday night.

“My family’s story isn’t special. What’s special is the America that makes our story possible,” Julian Castro was set to say, according to excerpts of his speech released Tuesday.

“Ours is a nation like no other, a place where great journeys can be made in a single generation. No matter who you are or where you come from, the path is always forward.”

Castro also criticized the economic policies of Republicans Mitt Romney and Paul Ryan.

“We all understand that freedom isn’t free. What Romney and Ryan don’t understand is that neither is opportunity. We have to invest in it,” he was to say.

Some would say the mayor has had a swift and charmed ascent. But his mother, whose own political activism on behalf of Hispanics when her boys were young drew hate mail and, she said, the attention of the Justice Department, knows her sons’ rise is evidence of Hispanics’ growing and long overdue political power.

“They called us militant, but our way of doing things was through political ends,” Rosie Castro said of her fight for Mexican-American rights in the 1970s. “Not through guns, not through overthrowing the government, but through the political process.”

How the twins even wound up in elected office is a debt owed to their mother. Now 65, Rosie Castro is the protagonist of the family story the mayor related to a national audience.

A civil rights activist and single parent from the time the twins were 8, she dragged the boys to rallies and often argued politics with them.

Julian Castro said he knows his mother’s generation faced different burdens than the Hispanics he is trying to connect with today, and he credits her civil rights work with laying the ground work for his political success.

“It was very warranted at that time,” the mayor said. “You had a huge dropout rate. You had signs that said, ‘No Mexicans or dogs allowed.’ It was a movement born out of both aspiration and frustration. It was very understandable. And ultimately, I believe, helped move this country forward.”

Rosie Castro made a failed run for San Antonio city council when she was 23, but quickly found her calling as a political organizer in Texas’ Mexican-American community in the 1970s. She didn’t reap the fanfare enjoyed by her sons.

Sitting in a downtown Mexican restaurant before leaving for the convention, Rosie Castro ticked off the costs of her activism — harassing phone calls in the middle of the night and packages sent to her house calling her communist.

She said authorities kept close tabs on her organizing efforts, and a local newspaper branded her as militant.