Then-San Antonio Mayor Julian Castro testifies on Capitol Hill on June 17, before a Senate Banking Committee hearing on his nomination to become secretary of housing and urban development. (Lauren Victoria Burke/Associated Press)

As she expands her political network in advance of an expected presidential run, Hillary Rodham Clinton and her husband have been cultivating an important ally who some believe could become her vice presidential running mate.

Former president Bill Clinton invited Julian Castro, a former San Antonio mayor and incoming Obama Cabinet secretary, to the Clintons’ home in Washington last week for a private dinner that friends described as a chance for Democratic leaders from different generations to become better acquainted.

Castro, 39, who is scheduled to be sworn in Monday as secretary of housing and urban development, traveled to New York in July to join Hillary Clinton, as well as Supreme Court Justice Sonia Sotomayor, at a children’s song and dance performance for the Bronx Children’s Museum’s youth arts program.

And in March, Hillary Clinton sat next to Henry Cisneros, who served in her husband’s Cabinet, at a private luncheon in New Mexico, where Cisneros said they discussed Castro and his political future.

“It’s a natural friendship waiting to bloom,” said Cisneros, also a former San Antonio mayor and a longtime family friend and political mentor of Castro and his identical twin brother, Rep. Joaquin Castro (D-Tex.).

Said another person familiar with the discussions, who spoke on the condition of anonymity so as not to alienate either camp, “The Clintons are keeping the Castros very close to them.”

The behind-the-scenes maneuvering illustrates how the Clintons are trying to acclimate themselves into a Democratic Party that has evolved and nurtured new stars in the years since they ceded the stage to Barack Obama in 2008.

For the Clintons, there are clear advantages to building an alliance with Castro. A young and dynamic figure who broke onto the national scene with his keynote address at the 2012 Democratic National Convention, Castro is arguably the only Hispanic Democrat with a broad following. Although his background as a Mexican American could have broad appeal to Hispanic voters, Castro does not speak fluent Spanish.

Assuming Clinton runs for president, keeping Castro and his brother on her side is key because any sign of wavering in their support of her candidacy during the Democratic primaries could complicate her attempts to court the increasingly influential Hispanic electorate.

Should Clinton win the Democratic nomination, Castro could find himself on Clinton’s vice presidential short list. Clinton may face pressure to select a Hispanic running mate, especially considering that the Republican Party could field two Latino presidential candidates, Sens. Marco Rubio (Fla.) and Ted Cruz (Tex.). Other Latino Republicans, including New Mexico Gov. Susana Martinez and Nevada Gov. Brian Sandoval, have been mentioned as potential vice presidential candidates.

“If Hillary Clinton is the nominee, there will be many considerations, but certainly one of them will be the next generation and another one will be the significance of the Latino community,” Cisneros said. “Clearly, if you were putting together a list of five people in the country who could potentially be a contributing running mate, you would have to put Julian Castro on that list.”

There are benefits for Castro, too, in establishing closer ties to the Clintons. During his third term as mayor, he resigned to join President Obama’s Cabinet, a move that close associates said could demonstrate national political experience he would need to be seriously considered for vice president. Even if Clinton bypassed him as a running mate, Castro could land a different high-profile post in her administration should she win or could run for statewide office in Texas with support from the Clinton network.

Castro met Bill Clinton at the family’s home on Whitehaven Street NW, just around the corner from the Naval Observatory, the vice president’s official residence, for dinner on Aug. 5, when Clinton was in town for the U.S.-Africa Leaders Summit. They were joined by Sandy Berger, a national security adviser in the Clinton administration, and other Clinton associates, said Democrats familiar with the dinner.

Although politics is always in the atmosphere at a dinner for politicians, aides to Clinton and Castro insisted that their discussion centered around joint initiatives between the Department of Housing and Urban Development and the Clinton Climate Initiative, one of several philanthropic programs affiliated with the Bill, Hillary and Chelsea Clinton Foundation.

“Secretary Castro and former president Clinton had a discussion about ways the agency can expand on the partnership with the Clinton Climate Initiative to make public housing more energy-efficient,” HUD spokeswoman Betsaida Alcantara said.

A Clinton aide, who spoke on the condition of anonymity, made a similar statement and then got to the point: “They didn’t talk about 2016.”

Berger did not respond to a request for comment.

A Clinton-Castro pairing has long been the subject of speculation in political circles. When asked in May about the prospect of running on a ticket with Castro or Sen. Elizabeth Warren (D-Mass.), Clinton told ABC’s Robin Roberts, “They’re both extraordinary leaders and great political advocates for a lot of what needs to be done in our country, and I admire both of them greatly.”

The Clinton-Castro relationship dates at least to 2012, when the former president and the Castro brothers appeared together at a political fundraiser in Los Angeles, shortly after Julian Castro’s DNC keynote speech. In his remarks at the event, Clinton suggested that Julian would one day be president and that the Castro brothers were building a legacy of public service similar to that of the Kennedys, according to a close associate of both camps who was in attendance.

Since then, the Clintons and Castros have found occasional opportunities to develop a relationship. In February, when Bill Clinton visited San Antonio for a speech to the World Affairs Council, he went to lunch at Mi Tierra, an iconic Tex-Mex restaurant in the city, with Julian Castro, who at the time was still the mayor, as well as Cisneros and San Antonio Spurs coach Gregg Popovich.

The Castro brothers were born in 1974. That was the year a 28-year-old Bill Clinton, fresh out of law school, ran his first campaign, for Congress from Arkansas, as a sort of boy wonder. Twenty years later, Julian Castro was an intern in the Clinton White House, working in the Office of Cabinet Affairs, and he has said he had his picture taken with the president.

“It may well be there’s a special affinity there for these early overachievers,” said Paul Begala, a longtime Clinton adviser. “President Clinton’s got an eye for talent.”

Bill Clinton also has a deep affection for Texas and many political friends there after he and Hillary crisscrossed the state in 1972 working for George McGovern’s presidential campaign.

In the decades since, Begala said, Clinton has gone to San Antonio whenever he has had a reason. The former president had a taste for mango ice cream from the Menger Hotel, a legendary property near the Alamo where Teddy Roosevelt recruited the Rough Riders.

“Somebody should tell Julian to ship a little mango ice cream up to him,” Begala said. Acknowledging the former president’s strict diet of late, Begala added, “I don’t know if it’s vegan.”