He’s plotted his rise carefully, studying and strategizing with a clear goal in sight. But if Clinton picks him to be her running mate, it’ll be more about perfectly fitting his party’s moment and the nearly non-existent Democratic bench than about his 18 months as a HUD secretary who hasn’t left a deep mark at his agency, the White House or the housing world.

Castro’s got a made-for-campaign commercials biography, and an undeniable savvy that’s helped him spin a job as a part-time mayor of San Antonio into an unusually successful 2012 Democratic convention keynote and then the answer to the second term Obama White House’s own search for more diversity in the Cabinet, without having to wait until he could win statewide in Texas.

Since he got to Washington in 2014, Castro’s invested in building relationships with key members of Congress, paying close attention to the people who have control of HUD budgets and who might make good political connections to have down the line. He’s used the allure of getting to hang out with a possible future vice president to make fans out of housing industry leaders who nonetheless have trouble citing anything specific they like about his work, streamlined a famously dysfunctional bureaucracy and walked the hallways to improve employee satisfaction so much that several HUD employees who rolled their eyes at his appointment now say he was a needed change.

Along the way, Castro’s inner circle of aides and advisers have come to see Clinton’s running mate decision as a pretty simple math problem: it’ll be a white man, a black man or a brown man, they figure.

And then they run it down from there: the white man’s Sen. Tim Kaine, and though he’s got a lot of good government experience and the geography that could help with Ohio and Pennsylvania on top of his own Virginia, they say that at 58 he’d be too old to provide a generational contrast to the 68-year old Clinton, and too white for a campaign being defined by immigration reform and Black Lives Matter and Donald Trump’s attacks on minorities. Yes, Kaine speaks Spanish—and fluently, from his days as a missionary in Honduras, as opposed to Castro, who like so many first-generation Americans, had parents who only spoke English with him—but they don’t think it’ll go over too well for Clinton to try explaining to Latinos that at least the white guy she picked over a Latino speaks their language.

If it’s a black man, they say, then it’s Deval Patrick or Cory Booker. Patrick’s out, the Castro circle figures, because Massachusetts doesn’t help Clinton and that Bain Capital job he took after leaving office is a killer. Booker’s possible, though maybe a New York-New Jersey ticket’s a bit much.

Castro declined comment on any of the gaming out. But ask the Castro circle if there’s any other Latino they can imagine Clinton picking and they stammer. Xavier Becerra, they say skeptically, their voices trailing off as they stretch a question mark out the last vowel in the California congressman’s name. And at 57, they figure, he’s also too old.

Castro’s a young-looking 41, a lower-level Cabinet secretary with a record that’s most distinguished for not having anything go wrong. He’s the former mayor of the seventh-largest city with a record that includes a big win in implementing universal pre-K, but the job is technically part-time and pays only $3,000 per year. (Most of the municipal government is run by an unelected city manager, though many San Antonians expect the mayor to act like they’re full-time.)

A Clinton spokesman declined comment on where the campaign is in the running mate selection, but to Castro’s backers, the choice is obvious.

Henry Cisneros, a Castro mentor and the other former mayor of San Antonio who became HUD secretary, under Clinton, said he pushes the idea of picking him to the former president and secretary of state every chance he gets.

“When you consider all of the balancing factors—female/male, baby boomer/gen X, traditional American/minority American, Northeastern-Midwestern orientation/Southwestern orientation, long heritage in office/representative of a new generation,” Cisneros said, “a lot of things suggest a very nice pairing.”

* * *

Bill Clinton first met Castro at a fundraiser for Hillary Clinton in 2008, in the San Antonio home of Henry Muñoz, now the DNC finance chairman. Four years later, Clinton dialed him up with advice about the convention keynote. Clinton invited him to a small lunch with Cisneros when he was back in San Antonio in February 2014, and then was on the phone again a few weeks after President Barack Obama nominated Castro for the HUD job, providing an intense 45-minute rundown on HUD policy, management and politics.

Then, a few weeks after Castro got to Washington, Clinton invited him to a dinner with friends at his D.C. home, without Hillary. Back then, Castro had only met her once, for a couple minutes backstage at a conference at the University of Southern California in late 2013.

Castro turned down the first two jobs he considered for the Obama administration.

He said no to Homeland Security before anyone ever offered it, after a Democratic operative with White House ties reached out to gauge his interest in taking over for Janet Napolitano. No way, Castro and his tight circle of advisers decided. The concern wasn’t that he had no national security or law enforcement experience, but, according to people familiar with the conversations at the time, because: what if there was an attack on his watch? That would have ended the political career he’d started with a fundraiser with his twin brother among his Harvard Law classmates before they graduated.

When Obama did call with an offer a few months later, Castro had already figured out what it was—Transportation secretary—and that he didn’t want it, though he and his advisers had been intrigued by the kind of travel he could do with the private jet that came with the job. You can tell Obama’s a good lawyer, Castro told people after he got off the phone, from how he felt Castro out without ever actually saying what the call was about. Castro couldn’t see what he was going to get from spending a couple of years cutting ribbons on infrastructure projects and talking up the Highway Bill.

Before they hung up, though, Castro pitched Obama on Education secretary, if and when that opened. That’s the kind of thing he felt like he could do something with.

Castro remembers very well what Walter Mondale told Cisneros during his 1984 running mate interview: it’s hard to go from being mayor right onto a national ticket.

“He believes,” said one person who knows him, “in being on the right platform.”

Castro looked hard at the 2014 Texas governor’s race, always wanting to be convinced that it was the right move, at the right time. He passed again, waiting.

A year and a half after that first call from Obama, Castro got another. He took the weekend to think about it, but he already knew the answer was yes. Within weeks, he was at HUD calling in new staff and holdovers, asking them for their vision for the year ahead. If they started rattling off about policy, he’d wave them off.

“Here’s the message,” he’d say, according to several people in those conversations. “HUD is the Department of Opportunity. And everything we do will be around that message.”

When detailed policy questions about housing and mortgage financing came up, those people said, he’d freeze up. He seems genuinely interested in helping people, they’d say, but not interested enough in HUD policy to want to engage deeply.

“People like Julián Castro and he’s not stopping them from doing their work,” said one person familiar with the Department. “He’s also not driving it. He’s not interested and engaged like that. But it’s hard not to like him.”

Sitting in his office on the tenth floor of the particularly brutal Brutalist HUD headquarters in Southwest D.C., a vintage poster from his mother’s 1970s San Antonio city council run framed on the wall behind him (having her on the campaign trail would be reason enough for Clinton to pick Castro, some fans argue), Castro said he wasn’t having fun at first. As mayor he was the center of attention, in charge of deciding what he’d do. This was a change.

“I’d be lying to you if I said that is there no part of my ego that likes it when people know that I did something,” Castro said. “People are bullshitting you if they say there’s zero percent of that involved. But I think that the main satisfaction that policy makers can have is understanding that what they do makes an impact on people’s lives.”

Castro’s no “houser,” as HUD people say, and he’s suffered in a lot of eyes from the comparisons to Shaun Donovan, the rare HUD secretary who truly came up through housing finance and policy and was known on the job for his seemingly endless policy brainstorming sessions that Castro’s scrapped. Instead of Donovan’s giant briefing books, Castro’s ordered they all come with one-page summaries with very clearly defined options.

To many around town, Castro’s reputation as HUD secretary was cemented by his first hearing in front of the House Financial Services Committee last February. Close Castro allies blame Republicans with 2016 on their minds for trying to discredit a potential vice presidential candidate the same way they’d used the Benghazi hearings to go after Clinton. But the faces of the aides behind him in told a different story: he was bumbling, confused on details and letting very eager Republicans ride all over him.

“It’s taken him some time to get up to speed,” said Rep. Sean Duffy (R-Wisc.), the chair of the Oversight & Investigations subcommittee on Financial Services.

Aides admit they fumbled by giving Castro too much information that hadn’t been boiled down enough for a relative novice, never resolving internal fights about how to present some of the dicey facts around housing finance, not realizing how often in tutorials he was just nodding along instead of asking questions.

But the real change came during the lunch break. Rep. Joaquín Castro (D-Texas) is his twin brother’s closest adviser, side-by-side at college and law school and in San Antonio politics—“they’re almost like their own political party together,” said one Democrat who’s seen them in action together — and that day he came close to shaking his brother: Push back. Get aggressive.

Castro was noticeably different in the second half of that hearing, and by the next one in June, he still didn’t know all the specifics, but he covered that by parrying and counterpunching, but mostly working the room before, during and after with phone calls, lunches and district visits.

He’d mastered the politics, if not the policy, of going before Congress.

“He is a cool drink of water in a desert of hostility,” said Rep. Emanuel Cleaver (D-Mo.).

There are other benefits, people who’ve been watching closely note. Cleaver, for example, is more than just the ranking member on Oversight & Investigations for Financial Services; he’s a prominent African-American Democrat with ties to the Clintons. Castro’s invested a lot of time in him.

Cleaver now says his “great hope” is that Clinton will consider Castro for VP.

“We made a conscious effort to cover our bases and make things move on the committees,” Castro said.

Duffy gushed about how much Castro’s engaged him, including taking an extra night in northwestern Wisconsin to have dinner with him before visiting the district in November.

“I’m a Republican, he’s a Democrat, a cabinet official, and I’m well aware being talked about as the vice presidential candidate. But putting partisan politics aside and coming out to my district, for me, that was a really big deal.”

“He’s a person,” Duffy said, “who is engaging and building coalitions and looking out for people in, I think, a unique way.”

* * *

Ask many people in the West Wing what they think about Castro, and the answer is: they don’t think about him much at all. They’re protective of him at the top levels of the White House, and sense his political potential, but he’s not one of the cabinet favorites in the building.

At HUD and the White House, people strain to think of many moments between the secretary and the president that stand out, aside from Obama inviting him to ride in his limo between every stop on a trip together to Arizona last January.

The one moment that does: when Obama came to HUD for the welcome ceremony after Castro was confirmed, and the secretary wouldn’t let him leave before he took a picture with the cleaning staff. Obama was touched by the idea, say people who spoke with him after, and impressed with the political sensibility it showed.

But the word that often comes up in describing their relationship is “cordial.”

In an interview at the end of last year, Castro said it had been “some time” since he’d spoken to the president, and that wasn’t unusual.

Castro said he’s gotten to know Obama “much better than I did when I was mayor. Am I his best friend? No.”

Castro was one of the three suggested replacements that Donovan gave Obama when he was getting ready to move over to the Office of Management of Budget. They liked the work Castro had been doing on Promise Zones in San Antonio—“You can see the needle moving for kids on the east side of San Antonio because of the way he organized his resources and federal resources,” said Cecilia Muñoz, Obama’s domestic policy advisor, and Castro’s main point of contact in the White House.

White House aides didn’t have any illusions about Castro. They knew he didn’t know much about housing, and without a deputy secretary in place to run HUD on a day-to-day basis for him, they vetoed his first choice for chief of staff, a political pick from back in San Antonio who also didn’t come out of housing.

Coming in six years into the administration, with nothing Obama wants coming out of Congress and the White House’s focus moved on after stabilizing the housing crisis, Castro’s been working in a narrow lane.

“It’s not like he comes to town and set the world on fire around housing. But it’s a very difficult environment to do that,” said Sheila Crowley, the president of the National Low Income Housing Coalition. “If the measure is progress and the addressing of affordable housing issues, there’s nobody who’s going to be able to point to many outstanding successes.”

Castro’s big contribution to administration policy, the mortgage premium reduction that was one of the spotlighted policy roll-outs for Obama’s 2015 State of the Union, was ordered up by the White House months before Castro arrived in Washington.

On that and top administration priorities like ending veteran’s homelessness, Castro’s had good results, White House aides say, though more as an implementer than an innovator—just the way this White House likes it.

“I don’t believe there is a Castro agenda,” said Obama senior adviser Valerie Jarrett. The president “wasn’t looking for someone with their own agenda, he was looking for someone who shared his agenda. Since his appointment, he has been delivering on the president’s agenda.”

Once Castro’s decided on an issue, though, he wraps his arms around it. He was the one who made the final call on who’d qualify for the mortgage premium reduction, and even HUD employees who think he’s out of his depth say he deserves credit for taking a chance that could have come back to bite him if the housing market had tanked at the end of last year.

Castro has expressed concerns to White House aides that they’re tilting too much to the banks in making rules about loans, not giving enough flexibility to consumers. But most of the decisions have already been made, and the White House has left the remaining ones mostly in the hands of Donovan, Treasury Secretary Jack Lew, National Economic Council director Jeff Zients and their teams.

The White House has also tapped Castro to advise and promote the White House’s work with mayors around the country on issues like paid leave and universal pre-K. He’s been “immensely helpful both in talking through the ideas and managing those relationships with the nation’s mayors since he knows the challenges and opportunities first-hand,” Muñoz said.

Castro and his team have embraced the freedom of not having to create a full agenda. Instead of staying in Washington, he’s on the road nearly every week promoting some aspect of what HUD is doing -- and building up press, speaking experience and connections for himself as he goes.

“His experience as a mayor far exceeds his portfolio for housing, because he understands what it takes to make communities healthy,” Jarrett said. “It isn’t housing in a vacuum, it’s housing where you have good education, safe streets, jobs, infrastructure.”

He’s taken advantage of being the new guy who can get his calls returned — getting Obama to be part of the rollout of his signature ConnectHome initiative to expand broadband access in public housing, playing a central role in putting out the Affirmative Furthering Fair Housing rule (which provides a long-sought mechanism for HUD to force anyone getting federal housing money to address discrimination), enjoying wide latitude over how he runs his relationships with Congress and the media.

“Was he pounding on the table at the White House?” said one sympathetic former HUD official. “No, but he’s a quieter leader, and he knows how to pull the levers together for what needs to get done. He’s very disciplined.”

* * *

Clinton’s greatest liability, people around her point out, is the sense that she’s willing to do and say anything to get elected. Wouldn’t picking a running mate who makes so much political sense but has such a thin résumé put a face on that feeling?

Then again, though Castro clearly wouldn’t be Al Gore, Dick Cheney or Joe Biden coming in, she’d have the experience argument covered herself, they say. Anyway, what are the chances Clinton’s going to give much power to her vice president?

But the decision could be more basic than any of that.

By the time Clinton’s making the choice, they say, she’ll be thinking there’s better than even odds she’ll be president, coming in at 69 as one of the two oldest presidents to take office. “What if I die?” is going to be a real question for her. Is the answer really going to be, Julián Castro’s the president?

Then again, her best-case scenario is a trudge through November with nearly every voter in the country already decided on her one way or the other. The primary’s proven that she doesn’t seem fresh or new, that there’s such an enthusiasm gap that an ornery socialist senator from Vermont has turned a symbolic challenge against her into a real primary challenge, and so some wonder, can she win without him?

Especially young Latino audiences go nuts over Castro, but for a guy whom Clinton would count on to electrify, he’s often not too electrifying — one of those people who’s almost easier to read and connect with in front of 10,000 people at a rally or on TV than in front of one, people who’ve worked with him say.

“All of us have this sense of a politician that’s stuck in the mid-20th century of the glad-handing, larger than life image,” Castro said. “Bill Clinton maybe gave a little oxygen to it. But if we step back and take a look at folks who put themselves up for office, rarely does it match that anymore. And why would it?”

But even the people who complain about Castro’s caution say it’s less him protecting his VP prospects than just never wanting to be out on a limb on anything. The good news, say people who know him, is that if he’s picked he won’t make rookie mistakes. The bad news, others say, is that they fear he’ll be too wary of taking chances to make any impression at all.

“Sometimes folks consider me overly cautious because of my personality,” he said. “You look at the track record of what I’ve actually swung for — that’s stuff that wasn’t being done before.”

Castro offered to endorse Clinton the third time he was ever in the same room as her, when they ran into each other at the National Council of La Raza convention in Kansas City in July. She wanted to wait, but when her campaign was trying to accelerate the end of Vice President Joe Biden’s 2016 flirtation, they quickly came calling. Standing in the back of the ballroom of the Congressional Hispanic Caucus Institute gala at the beginning of October, they sealed it.

Their event together in San Antonio, when Clinton said she’d “look really hard” at Castro, was the only time she’s said anything about any potential vice presidential pick. These days, all a campaign official would say is, “Hillary Clinton greatly appreciates the endorsement and support he’s shown for her campaign.”

Castro calls her “unflappable. Very knowledgeable. Just generally stronger on the stump and in those debates than anybody else, Republican or Democrat.”

“It’s fair to say I haven’t spent a whole lot of time with her, but the time that I have, I’ve had the chance to start to get to know her,” he said.

On the question of how he thinks Clinton would differ from Obama as president, he ducks. On the question of how he thinks his experience might fit into an administration, he ducks again.

When he’s asked about the vice presidency, inevitably, everywhere he goes, he smiles the same smile and slowly says his line about how he’s focusing on his job now, and that’s the best thing he can do.

Ask him whether America’s at a turning point moment for Latinos to get a bigger role in national politics, and he answers quickly.

“We’ve been at that moment,” Castro said, “for a while now.”