SAN ANTONIO - After years of being confused for his twin brother, Rep. Joaquin Castro (D-Tex.) has come up with an easier way for local residents to distinguish between himself and former mayor Julian Castro.

"If you see one of us here, it's me. There's one of us left in town," he told a meeting of local nonprofit leaders last week.

Castro the congressman is the only brother duty-bound to spend time in Texas now that his brother is serving as President Obama's secretary of Housing and Urban Development -- and he's embraced that mandate. Secretary Castro is frequently mentioned as a possible 2016 vice presidential candidate. The congressman is increasingly focused on the politics of his home state.

He doesn't deny interest in running in 2018 against Sen. Ted Cruz (R-Tex.), or for governor if Democrat Wendy Davis loses this year, as expected, to Republican Attorney General Greg Abbott in the race to succeed GOP Gov. Rick Perry.

Even as he's emerged as a frequent spokesman for congressional Democrats -- especially on immigration reform -- he is determined this year to use his family's budding national profile and fundraising prowess to help Texas Democrats by visiting far-flung pockets of the state, including communities where being a Democrat can often feel desert-island lonely..

"I want to go to places in the state where Democrats don't normally make their case," he said in an interview. "I know that hopefully some people will agree - and there will be lots of people who disagree. But I think people appreciate when you make the case."

The state's shifting demographics -- a fast-growing Latino population and an influx of residents from other states - make political change inevitable, political observers say -- especially as the GOP continues to grapple with how to address immigration reform, a top concern for Hispanic voters.

But building Democratic support in Texas is a herculean struggle because Democrats -- or people who probably would vote for Democrats -- haven't been showing up to vote in recent election cycles.

"I think Democrats are climbing out of a pretty deep hole here, but I don't think anyone would question that they're climbing, not digging," said Harold Cook, an Austin-based state political pundit.

“The path to a purple Texas, and eventually a blue Texas, is through changing voter behavior,” said Jeff Rotkoff, a Democratic operative. Right now, “our population does not vote,” he said. “…They just sit out elections. One of the reason I believe they sit out is that nobody’s knocked on their door.”

San Antonio businessman Henry R. Muñoz III, who is national finance chair of the Democratic National Committee and has known the Castro brothers since they were young boys, said the congressman is uniquely positioned to help.

“He is, separate from his brother, an astute, emergent leader in his hometown, within his district, across the state of Texas and you only need to look at the Sunday news programs to understand that he’s got a voice that’s listened to nationally,” he said.

It's why Castro is reaching out to people like Leo Flores, a Democratic Party volunteer in Lubbock, Texas. One night in October 2012, Flores caught local college students on camera ripping up campaign signs for Obama's reelection campaign. When he confronted the students with the footage, they attacked him and yelled racial slurs.

The incident caught the attention of local police, the local press and Castro, who was campaigning for his congressional seat almost 400 miles away in San Antonio. He picked up the phone and called Flores.

"He lifted my spirits up," Flores recalled. "He told me to stay strong, keep doing what you're doing, work to make this a better place for everyone and to keep marching forward." Then Castro promised to eventually visit Lubbock and help raise money for the county Democratic Party.

"It was like a blessing from the skies," Flores said. "Being in West Texas, it's hard to let people know you're a Democrat… He rejuvenated a lot of people who were losing hope."

Making good on his promise, Castro traveled to Lubbock this past March, nearly a year and a half after the incident. He invited Flores as his guest and helped the county Democratic Party raise nearly $10,000 – a paltry sum for most political organizations, but “that’s money we wouldn’t have been able to raise otherwise,” said Kenny Ketner, the Lubbock County Democratic Party Chair. "He let us know that we weren't forgotten out here in Lubbock, and that we can win in areas out here."

Castro did it again in Midland, Texas, where George W. Bush got his start in politics. It's a Republican stronghold, but Castro showed up at Martinez Bakery in March and met with about 50 local Democrats. “He came out to raise the Democratic flag, to meet us, to encourage us. It was a big get out the vote effort,” said David Rosen, chairman of the Midland County Democratic Party.

And he's been to areas around Dallas and Houston and to Victoria, Fort Bend and Odessa counties -- rural spots that currently offer little hope for local Democrats. The work in the smaller counties has resulted in more than $132,000 raised for local party operations. With tens of millions of dollars pouring in to the high-profile Davis-Abbot race and little left for down-ballot candidates, “that’s a sizable chunk of change for the local parties,” said Will Hailer, executive director of the Texas Democratic Party. "I've never seen a congressional member so involved in helping raise resources, especially at that level to a state party."

For a freshman lawmaker, Castro has also kept a busy national political travel schedule too. Since becoming a congressman last year, he's made stops in Iowa (with his brother), South Carolina, California, New Jersey, Maryland and Indiana, to help state and national Democrats raise millions of dollars -- and presumably, score some favors for his own future. More stops in California, Florida and Washington state are coming soon.

Still, he’s not currently thinking about national office – right now, that’s the path for his brother. The mayor’s move to Washington is widely seen by close associates and longtime Texas political observers as a way to put him in the running as a potential vice president – or more senior cabinet official – in a future Hillary Clinton administration.

Dick Harpootlian, a former South Carolina Democratic Party chairman, recalled that Castro was “a nice young man” who spoke at a party fundraiser in the state last fall. When asked whether he sensed any national ambition from the lawmaker, Harpootlian said: “Absolutely not.”

Despite his travels across the country and Texas, Castro is a welcome presence for Democrats on Capitol Hill. His staff includes aides who once worked for Senate Majority Leader Harry Reid (D-Nev.) and House Minority Leader Nancy Pelosi (D-Calif.) — plum hires for a first-term lawmaker. He’s earned praise from Pelosi and the Obama administration for helping local leaders enroll nearly 70,000 San Antonio-area residents for health insurance last year. The “Enroll SA” program has been widely credited as a national model for how other communities can drive up awareness and participation in the new health care law.

Castro came to Washington last year as part of a wave of new, younger Texas Democratic congressmen. There’s Rep. Pete Gallego (D-Tex.), whose border district stretches from San Antonio to El Paso and is in the state’s only competitive congressional race this year. Reps. Beto O’Rourke, of El Paso, Filemon Vela, from Brownsville, and Marc Veasey, who represents parts of Fort Worth, also were elected in 2012 from safe Democratic seats, but haven’t sparked as much interest statewide or in Washington. (Rotkoff, however, talked up Veasey. “If Marc had a twin brother who was mayor of something, we’d be talking more about Marc too,” he said.)

Castro, however, stands above the rest. Back in San Antonio he's easily recognizable, but still often misidentified. He's frequently called "Mr. Mayor" and more recently, "Mr. Secretary." Online, people often use the incorrect Twitter handle to identify either brother.

One night last week at Rosario's, a Southtown neighborhood Mexican restaurant where he usually orders the Enchiladas de Mole, Castro shared dinner with his wife, Anna, an aide and a visiting reporter. He wanted to keep the conversation focused on the broader politics of Texas - not his own political future. "It would be something that makes sense," is all he would offer on the record.

At one point he was approached at the table by a couple who said they live in West Texas.

"Mr. Secretary! So nice to meet you, you're going to have to come to West Texas," the wife told him.

"Oh well, hi, but I'm the congressman," he told the couple. "I've been a few times, and I want to go back."