For the second time in two years, President Barack Obama has offered Julián Castro a chance to serve in his Cabinet, and the mayor has signaled his willingness to begin a swift process of confirmation to the post, knowledgeable sources say.

The process includes a vetting of Castro by the FBI — which has begun — and a Senate confirmation hearing, expected to conclude within months.

Castro, whose mayoral tenure thrust him into the national spotlight, refused to comment Friday. It was unclear what post the president has offered the Democratic stalwart.

Castro’s departure from San Antonio for the nation’s capital, where he would join his twin, Rep. Joaquin Castro, would come five years after he first was elected mayor, and one year before he could run for re-election to a final two-year term at City Hall.

Obama gauged Castro’s interest in serving as transportation secretary last year, but the mayor declined.

Publicly, Castro has said he plans to serve as mayor here as long as the voters would have him. In private conversations, though, he’s said an offer from the president to serve as education secretary would have proven tougher to turn down.

Also tough to turn down is a chance to run as nominee for vice president alongside Hillary Clinton.

For months, Castro has dismissed speculation that Clinton could pick him as a running mate if she becomes the Democratic nominee for president in 2016.

“I don’t want to play the speculation game,” Castro told San Antonio Express-News reporter Josh Baugh last year. “There’s not a person in America who can predict what’s going to happen in 2016.”

Naturally, speculation has only grown. Last week, ABC’s Robin Roberts asked Clinton whether she’d consider Castro or Sen. Elizabeth Warren, D-Mass., as her running mate.

“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,” she responded.

For Clinton’s admiration to mature into confidence, though, Castro first must master a job greater than mayor. His willingness to resign his mayoralty early and move to Washington reflects this fact.

The national spotlight already has lingered on the mayor for months.

In 2012, Obama chose Castro to deliver a keynote address at the Democratic National Convention, making him the first Latino ever to do so. The mayor hit the campaign trail soon after as an Obama surrogate.

He opted not to run this year for Texas governor or lieutenant governor.

The president’s offer last year for Castro to join his Cabinet was poor timing: The mayor was on the cusp of seeking re-election to a third term at City Hall.

Former Mayor Henry Cisneros, a mentor to Castro who accepted an offer to join President Bill Clinton’s Cabinet after his own mayoral tenure, disapproved at the time of Castro’s decision.

Cisneros served as secretary of housing and urban development from 1993 to 1997 and was interviewed for a spot on Walter Mondale’s ticket in 1984. Mondale opted, though, for the first female nominee, Geraldine Ferraro.

“I advised that he accept a position for President Obama,” Cisneros told the New York Times. “I thought that if he was going to be vice presidential material in 2016, then he needed to be more than mayor at that time.”

In whatever capacity Castro hopes to assist Obama, it’s more than mayor of the nation’s seventh-largest city. And if he truly does become vice presidential material, he also has demographics to thank.

A record 11.2 million Latino voters cast ballots in 2012. That year, Obama won about 71 percent of the Latino vote on his way to a second term. Castro’s own heritage is an attractive dynamic as the Democratic Party looks for stars to guide it to victory in 2016.

Where does this leave San Antonio?

The City Charter has a preliminary answer.

“In the event of a vacancy in the office of the mayor, the remaining council members shall elect from among themselves, by majority vote, a person to serve as mayor for the unexpired term,” it states.

Let the jostling begin.

bchasnoff@express-news.net