San Antonio Mayor Julian Castro (L) and his brother State Rep. Joaquin Castro (D-TX) give an interview during preparations for the Democratic National Convention at Time Warner Cable Arena on Labor Day in Charlotte. (Streeter Lecka/GETTY IMAGES)

Julian Castro will step onto the national stage Tuesday night bearing a heavy burden of high expectations.

The mayor of San Antonio, plucked from relative obscurity for a keynote address on the first day of the Democratic National Convention, will be introduced to millions of Americans as the Hispanic version of Barack Obama — eight years after then-state Sen. Obama delivered his breakout convention keynote in Boston.

Or maybe Castro will be cast as the Democratic Marco Rubio, the Republican senator from Florida who has emerged as the country’s most visible Hispanic politician.

In any case, Democratic leaders hope the charismatic Castro, 37, will shoot to instant political stardom, particularly among Hispanic voters.

“You’re in for one of those moments that 10 years from now, you’re going to say, ‘I was there when he gave that speech,’ ” said Jim Messina, Obama’s campaign manager. On Monday, Messina told Hispanic delegates gathered for a strategy session that he had seen a draft of Castro’s remarks.

But even as Obama aides were hoping that the young mayor would provide some opening-night sizzle at the convention, they believe that the president has solidified his support among Hispanic voters, thanks to several actions he took over the summer.

Most prominently, campaign officials and convention delegates say, Obama’s June executive decision to allow many children of illegal immigrants to remain in the United States galvanized Hispanic voters.

“It just motivated everybody to think, well, maybe Obama didn’t do anything about immigration reform, but he did what he could by himself,” said Emma Sepulveda, a native of Chile who lives in Reno, Nev., and helps the campaign with Hispanic outreach in her home state.

The Obama campaign has been blanketing Spanish-language airwaves with ads promoting the immigration action as well as the president’s health-care overhaul.

Campaign pollster Sergio Bendixen drew applause from Hispanic delegates for a PowerPoint presentation of internal data showing Obama improving his standing over the summer in three Hispanic-heavy battlegrounds — Nevada, Colorado and Florida.

In Nevada, Bendixen said, Obama has boosted his share of the Hispanic vote against Republican Mitt Romney from 59 percent in April to 74 percent in July. (Romney dropped from 23 to 18, according to Bendixen.) There is little public polling to corroborate the campaign’s claims, though an NBC-Marist poll from May showed Obama beating Romney 61 percent to 33 percent among registered Hispanic voters in Nevada.

“We’re gaining,” Bendixen told the delegates. “Hispanics in these states basically like Obama a little bit more than they did in April.”

Obama won about two-thirds of the Hispanic vote in 2008, compared with 31 percent for Republican John McCain. Many GOP strategists worry that Romney, by staking out a hard line against illegal immigration, has alienated swing Hispanics from his party.

Romney’s campaign has bought substantial advertising time for hard-hitting Spanish-language spots expected to accuse Obama of broken promises on immigration and blame him for economic conditions that have left millions of Hispanics in poverty.

“Our research indicates that we’ve got significant potential for gains among Hispanics,” said Neil Newhouse, Romney’s pollster. “They respond quite negatively when reminded of the difference between President Obama’s promises and reality.”

Part of the Democrats’ response will be the San Antonio mayor.

On one level, Castro makes little sense as a keynote speaker. Unlike Obama circa 2004, who was headed to the U.S. Senate and was already known, Castro is mayor of a mid-size city with no clear path to statewide or national office. He hails from a solidly Republican state, albeit one with demographic trends seen as beneficial to Democrats years down the road.

But Democratic officials say they expect a good speech — “electrifying,” predicted the party’s national chairwoman, Rep. Debbie Wasserman Schultz (Fla.). Castro’s youth could inspire younger voters, another critical Obama constituency. Castro, the son of a single mom who went on to earn degrees from Stanford and Harvard, can tell a compelling personal story.

And many see him as the future in a party that lacks Hispanic star power. He and his brother, Joaquin, a Texas state lawmaker running this year for Congress, will try to fill a role that, in recent years, has been filled almost exclusively by older figures such as Los Angeles Mayor Antonio Villaraigosa and former New Mexico governor Bill Richardson.

“We all love Villaraigosa and Bill Richardson, but Julian Castro is someone who’s going to be around for the next 20-something years,” said Andres Ramirez, a Las Vegas-based Democratic strategist and vice chairman of the Democratic National Committee’s Hispanic caucus.