Former Obama administration official Julian Castro on Sunday struggled to define how his background qualifies him to be the next president of the United States.

Castro, a former mayor of San Antonio and President Barack Obama's secretary of Housing and Urban Development from 2014 to 2017, spoke on ABC's "This Week" about his potential 2020 presidential bid.

Anchor George Stephanopoulos, noting Castro's lack foreign policy experience, asked him what he believes is the "greatest national threat" and what qualifies him to tackle the issue.

"Well, I believe that today, the greatest threat to our national security is the fact that this president, as one of your previous guests has said, is damaging the relationships that we’ve had in place in the post-World War II era, whether it's NATO or other alliances with individual countries that have kept us safer," Castro said. "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."

Stephanopoulos pressed Castro on what makes him qualified to be the next 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 out there and make the case," Castro said.

Castro tweeted out a graphic earlier in the week letting his followers know that he would be making an announcement on Jan. 12 regarding his 2020 presidential plans.

"Join me in San Antonio on Saturday, January 12 for a special announcement," Castro tweeted.