Transcript for Julian Castro: Greatest national security threat is Trump damaging ally relations

I have never seen in my future that. Why not? Well, I think different things. I don't have a passion for that, you know. That's not -- Passion for? To try and become president. I'm not running for president. I guarantee you I'm not going to run for president. This year? Probably ever. That was Julian Castro a few years back, and the former San Antonio mayor and cabinet secretary is heading to Iowa and Nevada planning a big announcement on his presidential plans. He joins us live from San Antonio. Thank you for joining us this morning. Everybody has a right to change their minds, to find their passion. What's changed for you? Well, a lot of things have changed, George. You know, this country has changed a lot in terms of its leadership since 2015 or 2016 or whenever that interview was. I think there are a whole bunch of people that feel a tremendous difference in terms of the leadership that we had under Barack Obama versus the lack of leadership and the total disaster that we have under Donald Trump, and for me, I believe that I have a strong vision for the country's future. I also for starters, have run a federal agency and have been mayor of one of the biggest, most diverse cities in the country, and so I feel like I have something to offer, and -- As you know, it's going to be a crowded field on the democratic side. It's going to be hard to stand out. You have seen Elizabeth Warren. She has big plans to break up monopolies, reform capitalism, governor of Washington, and she wants to focus almost entirely on climate change. How will you distinguish yourself if you indeed do get in, and what do you have that the others don't? Yeah. Well, number one I have experience actually running one of these federal agencies being in charge of folks and making things work. Also being mayor of a city that is one of the most diverse cities as I said in the united States, and then really in a fundamental way, represents the diverse future of America, but I'm not going to be a single issue candidate if I run. I believe that what we need for America to prosper in the 21st century is a strong vision for the country's future. My vision for the country's future is we aim in the 21st century to be the smartest, the healthiest, the most fair and the most prosperous country that we need to invest in brain power because brain power truly is the new currency of success. Like universal pre-k, and hire -- universal higher education so people can get employment in a 21st century where our economy is changing. We need to be the healthiest and support things like medicare for all and make universal health care happen in the richest nation on Earth. I think that we need to be the most fair country. It is a shame today that we have not seen a minimum wage increase in almost a decade. It's also a shame that if you are a young, black man walking on the streets of many cities, that you are treated so differently than if the color of your skin is white, and I think that we need to be the most prosperous nation for everybody. It is clear. I like this about senator Warren. I think she is talking about a lot of good issues. It's different than it used to be, right? You used to be able to work 40 hours even on minimum wage and be able to provide for your family. Today that's not true, and we need to get back to a place in this 21st century where that is true. The economy works for everybody. You laid a lot of programs and one of the questions is how will you pay for them? Let me show you the youngest member of the coverage right now, Alexandria ocasio-cortez, and this is what she said, how to pay for things. You look at our tax rates back in the '60s and when you have a Progressive tax rate system, your tax rate, you know, let's say from zero to $75,000 may be 10% or 15%, et cetera, but once you get to your 10 millionth dollar, sometimes you see tax rates as high as 60% or 70%. Can you support a tax increase like that once you hit $10 million? Oh, I can support folks at the top paying for fair share. As you know, George, there was a time in this country where the top marginal tax rate was over 90%, even during Reagan's era in the 1980s it was around 50%. So do I support in order to have something like medicare for all, that we ask folks that are in the top .05% or .5% or top 1% to pay more? Also, that we get more serious about making sure the corporations pay their fair share, and that we're smart about understanding how instead of folks having to pay sky high premiums to companies that are seeking a profit to deliver health care that we can have a better system where people can get good health care and have peace of mind even if that means that we rearrange where those dollars go, yeah. I support that. You know what? During this campaign if I run, I'm going to be very up front with the American people on how we would do that because I think that they are owed that, but it is worth it. It is worth it in this country for us to do that. You laid out your experience and one thing you don't have is foreign policy experience. What do you see as the greatest threat to our national security today, and what qualifies you to take it on? I believe that today the greatest threat to our national security is the fact this this president as one of your previous guests has said is damaging the relationships that we have had in place since the post-world war II era, whether it's nato or other alliances with individual countries that have kept us safer. The first thing that I would do if I were president with regard to our relationships around the world is to strengthen them because those alliances have helped keep us safe. It's also true that today being -- being the strongest country, being the safest country I think requires more diplomatic efforts than ever, and it has been terrible to see the decimation of our diplomatic efforts through the state at the present time. We have a whole bunch of ambassadorships that are not even filled right now. We have vacancies throughout our diplomatic core. We have a harder time today recruiting folks to go through the diplomatic core. The number one thing that I would do is strengthen all of that so we could avoid conflict and be safer in the 21st century. That's what you would do, but what in your background qualifies you to be commander in chief? Well, I -- as I said earlier, I think that being mayor of a large city and serving in the president's cabinet certainly qualifies one to be commander in chief, and I'm going to go up there and make the case. Julian Castro, thank you for your time this morning. Good luck. Thank you, George.

This transcript has been automatically generated and may not be 100% accurate.