Correction appended

Julián Castro is potentially on the verge of the biggest moment of his life: being tapped to serve as the running mate for Hillary Clinton, the presumptive Democratic presidential nominee. 

One hang up, according to reports, may be his experience, or lack of it. The 41-year-old Castro, long regarded as a rising star in Democratic politics, has a relatively thin resume for a vice presidential prospect, serving five years as mayor of San Antonio before being plucked out of the job by President Barack Obama to lead the U.S. Department of Housing and Urban Development.

Now Castro is on the vice presidential short list for a nominee who has called her running mate's ability to fill her shoes the "most important qualification."

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Fiercely protective of his legacy, Castro's supporters chafe at the suggestion he is not qualified to be vice president. They acknowledge the obvious — he has little to no foreign policy experience — but argue he is the living, breathing embodiment of an American Dream that transcends mere lines on a resume. 

"He’s the West Side kid who went to the Ivy leagues, came home and not only did well, but changed the trajectory of the city," said Diego Bernal, a state lawmaker who has known Castro since he was 10. "Find me another person anywhere who’s done that and I’ll shake their hand."

Added Christian Archer, a longtime political consultant for Castro: "If you look at where he started and where he is today, are there 100 Americans who've accomplished what he's accomplished?"

Historically, vice presidential picks have had far more experience at higher levels of government than Castro does, according to research done by Joel Goldstein, a St. Louis University professor who specializes in the vice presidency. Goldstein found that running mates from the two major parties since 1976 spent an average of 14.5 years in one or more positions: U.S. senator, U.S. representative, governor or high officer in the executive branch.

Castro has been at HUD for just under two years. 

"I don’t think there’s any question that Secretary Castro is on the very short end of the experience ladder relative to other people," Goldstein said, adding that recent history shows experience tends to trump other factor in selecting a running mate. "If you pick somebody who’s not ready for prime time, the fact you’re from a particular demographic group or a big state — it’s not going to help if people can’t see them sitting in the Oval Office." 

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Castro was first elected San Antonio mayor in 2009, a few years after narrowly losing his first bid to retired judge Phil Hardberger. Castro decisively set out to learn from the defeat, setting up a law practice to gain private-sector experience, reaching out to a business community that had been skeptical of him and seeking out political advice from old foes — including Archer, who had been Hardberger's campaign manager. 

Ask Castro allies what his biggest accomplishments were in City Hall, and they all point to Pre-K 4 SA, a successful effort to create a full-day, early education program. It was a political risk, requiring to Castro to ask voters for a tax increase to fund a program that would not go fully into effect until long after he left office. 

"He went across the aisle," recalled Frank Burney, a San Antonio lawyer-lobbyist and Castro ally. "The easy ones were his own friends, but he went to the business community and got them to buy into this tax."

Beyond Pre-K 4 SA, Castro's boosters cite other education initiatives — including Cafe College, a kind of one-stop shop for helping city students apply to institutions of higher learning. His supporters also put a lot of his accomplishments in the context of SA2020, a long-term planning effort. 

Castro no doubt had detractors, including those who complained that he focused too much on San Antonio's urban core — he had endeavored to create a "decade of downtown" — versus other parts of the city. And especially after his well-received speech at the 2012 Democratic National Convention, he got flak for prioritizing his political ambitions, a charge that inevitably followed him to HUD.

After leaving for the Cabinet post, Castro was replaced at City Hall by council member Ivy Taylor, a more moderate Democrat who quickly oversaw the end of one of his signatures projects — a downtown streetcar. Vying for interim mayor in 2014 — and the permanent post a year later — Taylor touted a back-to-basics approach to municipal government that, at least to some of her supporters, sounded like an implicit repudiation of the Castro era and all the national attention it drew. 

Hidden in the cabinet

The official HUD mission is "to create strong, sustainable, inclusive communities and quality affordable homes for all."

As soon as Castro arrived in Washington in 2014, he was a visible novelty. But star power or not, HUD is an obscure Cabinet department, especially compared with the likes of the Justice Department and the State Department.

That low profile means Castro has avoided polarized policy debates and disqualifying crises. But it also means his visibility rarely sprang from his stewardship as a Cabinet member. Instead, interest centered on the allure of having an identical twin brother in the U.S. House, and to his future prospects for this very day: vice presidential nominee contention.

Outside of Washington, though, he earned high praise from an Austinite involved in housing. 

"The secretary came into office and began work on a streamlining rule ... and it’s making a difference in helping to simplify the way we do business and to help us be more effective," said Micheal Gerber, the CEO of the Housing Authority of the City of Austin. 

"He’s interested in making complex programs work," Gerber added. 

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One of the more high-profile moments of Castro's tenure came a few months ago, when progressive groups unleashed a wave of criticism against him, charging that his policies favored Wall Street banks over struggling families.

Castro recently pushed back on the suggestion that the campaign of Clinton's primary rival, Bernie Sanders, was behind the criticism. But, speaking with reporters last week in San Antonio, Castro said he did believe "that in some quarters there were politics involved."

“I give credit to a couple of the groups for bringing that issue up that they have brought up before," Castro said, "but I think that there’s a way that folks will work together in a positive direction on that."

Members of Congress both oversee Castro's department and advocate to him on behalf of their districts' housing needs.

Vice President Joe Biden, formerly a longtime senator, is more popular among House Democrats than the president himself — and they have similarly high expectations for Clinton's pick. 

In dozens of interviews with Capitol Hill Democratic members and staffers, the most frequently cited Castro drawback was that he lacks Biden's relationships and Congressional fluency.  

A non-Texas Democrat in Congress said that while at HUD, Castro's department did not answer several of this member's letters. When asked how frequently Cabinet members respond to Congressional queries, this member said: "Mr. Kerry's office does."

"His job now is to be responsive," said the Democrat, who requested anonymity so as not to get crosswise with Clinton should she pick Castro. "And particularly, if he got that [vice presidential] appointment, it would be to be responsive. Biden is responsive." 

Star of a small stage?

Among the biggest knocks on Castro's time as mayor is not one particular initiative, but the inherent limitations of the job in San Antonio. The city has a council-manager system of government in which an appointed city manager, not the elected mayor, holds most of the power.

Yet City Hall watchers credit Castro with leveraging the bully pulpit more than many of his predecessors did, bringing something of a balance to the relationship between his office and that of the city manager, who at the time was Sheryl Sculley. 

"He dominated City Hall when he was there, and he must be characterized as a strong mayor in a weak form of mayoral government," said Richard Gambitta, a retired political science professor at The University of Texas at San Antonio. "He always made it apparent he had the majority of city council on his side." 

Castro allies concede that foreign policy is the biggest — and most glaring — hole in his resume. But some note it may not be as relevant a credential as usual for whomever Clinton chooses as her running mate, given her deep experience as a former secretary of state and U.S. senator.

Republicans have tagged Castro as a lightweight ever since his name first came up as a potential running mate. Their thinking: Castro's resume is so thin that if Clinton picked him, it would have to be more for political than substantive reasons. 

"The hype about Secretary Castro may be a mile wide, but his policy substance is an inch deep," said Amelia Chassé, a spokeswoman for America Rising PAC, an anti-Clinton group that has been digging into Castro's background for more than a year. "Secretary Clinton has stated her 'top priority' for a vice president is readiness to be commander-in-chief, so by that definition alone, Castro is disqualified."

Chassé added that "it will be a sad state of affairs if Secretary Clinton’s political expedience forces her to put someone so completely unqualified one heartbeat away from the presidency."

Amid the months of buzz about his vice presidential prospects, Castro has studiously avoided stoking speculation. Asked a year ago if he had enough experience to be vice president, Castro ignored the question.

Someone who knows him well, however, has been more than happy to answer. 

"The fact is, my brother did more in a week on the San Antonio City Council than most members of Congress do in a whole year," Joaquin Castro said last year as his brother prepared to endorse Clinton. "He’s got a long list of accomplishments that we’re very proud of."

Correction: An earlier version of this story incorrectly reported the range of time in which St. Louis University professor Joel Goldstein studied the experience of vice presidential candidates from the two major parties.

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