Des Moines — In 2012, the Democrats tapped San Antonio Mayor Julián Castro, then 37, as the keynote speaker for the party's national convention. Nearly seven years later, the 2020 Democratic presidential candidate told CBS News that being in the spotlight never fazed him.

"I never really felt as much pressure," Castro said before pausing to gather his thoughts. "I think part of being young, when you're a rising star, is that you don't really feel that right away. You don't have much to lose."

For Castro, the real pressure came when he served as Secretary of Housing and Urban Development during the Obama administration. "You feel pressure in service to the person you are serving, the president, and to the American people," he said.

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For all the attention lavished on him by the national Democratic Party in the Obama years, critics say Castro's resume is thin. Mayors don't have much power in San Antonio, which is mostly run by an appointed city manager, and HUD is not a typical launching pad for the presidency. The last cabinet secretary to win the presidency, in fact, was Herbert Hoover in 1928.

When caucus goers bring up his lack of experience as he barnstorms across Iowa, Castro refers to his tenure at HUD as the most relevant time in his career. In Muscatine earlier this month, Castro responded to one caucus goer's question by talking about his "federal executive experience."

"I actually have stronger experience than many of the other candidates that are running because I have direct executive experience," Castro told CBS News. "That is what you are as the president."

Going into this week's second Democratic primary debate, Castro says he doesn't want to be "a flash in the pan candidate," and isn't looking for "a huge spike in support in the summer of 2019." He told CBS News he wants to be "in the right place at the right time" as the election season speeds towards the Iowa Caucuses.

"You've got to fight your way to be number one," Castro said about winning support in a field with 24 other candidates. He told CBS News he felt that people liked his performance in the first debate and said if he has a similar showing Wednesday night, people will "become more and more confident and convinced that I'm their candidate."

Earlier this month, Castro said he received over $1.5 million in donations since the first debate. In the latest CBS News Battleground Tracker poll of Democrats in states with primaries and caucuses through Super Tuesday, 2% of respondents said Castro was their top choice to be the nominee, putting him in 7th place.

In the first debate, the big moment for Castro came when he sparred with fellow Texan Beto O'Rourke over whether crossing the border illegally should be a criminal or civil offense. Castro, who believes illegal border crossings should be a civil offense, expects to face more questions about that issue on the stage Wednesday night, but he said it's "fair for people to point out legitimate policy differences."

"I was happy that in the first debate the disagreement that I had with Congressman O'Rourke was a policy difference," Castro said. "What we don't need to get into though is anything personal or a back and forth that is only noise and not really speaking to the differences people have."

Castro stressed that Democrats want to have a secure border, and highlighted the planes, helicopters, fencing and security cameras currently present along the nearly 2,000 mile stretch from the Pacific Ocean to the Gulf of Mexico. But since the issue of removing part of Section 1325 of the Nationality Act, which criminalizes illegal border crossings, came up at the last debate, he's remained adamant that changing the rule wouldn't benefit criminals.

"We have laws against human trafficking, laws against weapons trafficking, laws against drug trafficking," Castro said. "People have asked, 'well if you take away 1325, is that going to encourage more people to come?' And the answer to that is 'no.' For 60-something years, we didn't enforce that section of the law and that's not what either deterred or didn't deter people from coming to the United States."

On the campaign trail in Iowa, caucus goers have pushed Democratic candidates to be specific about their vision for health care and whether there is a role for private insurance. Castro says he supports a system that offers a public Medicare plan, but keeps private insurance.

"I see it more as a private option," Castro said. "If your plan is solid and you're comfortable with it then you should be able to hold onto it."

Castro also dismissed concerns that a system which includes both publicly administered healthcare and private insurance could create division if private insurance offers better care that some people cannot afford.

"We're going to have to create a system based on Medicare that is robust enough, strong enough, to provide very good healthcare," Castro said.

Castro said that if he is elected he wants to accomplish something that Mr. Obama could not: closing the Guantanamo Bay detention center. He acknowledged the difficulty in doing so, saying it would take "an education campaign" about what that would look like and says "work still needs to be done in Congress."

And regarding America's longest war, Castro said he would like to see the U.S. withdraw from Afghanistan, but maintain a small force with a "limited role." Another round of discussions between Trump Administration and the Taliban is expected this week, but peace talks have yet to include the Afghan government. Castro said the Taliban's role in a post-war Afghanistan needs to be addressed in negotiations.

"I think it's a matter of also ensuring that we stand up for our values like the role of women in their society," Castro said, adding, "my hope is that those talks will include how we can ensure that there is stability there by addressing a number of issues that may not have been addressed as robustly in the past."

When Castro takes the debate stage in Detroit Wednesday night, it will be his sixth day on the road. The father of two young children, Carina who is 10 and Cristián who is 4, says he tries to cap his time away from home at four or five days at a time.

"Especially with my son, who is 4. He misses me a lot and it is hard to be away from both of them and to be away from Erica," he said, referring to his wife.

Castro said he tries to Facetime or call his family on the nights when he is away and spend a day or two at home in between trips.

"That's the hardest part of the whole thing," Castro said.