Former Rep. John Delaney (D-Md.) and Rep. Tulsi Gabbard (D-Hawaii) have already announced definitive intentions to run for president in 2020. Sen.
Castro is the fifth Democrat to formally announce plans to enter the party's presidential primary race in 2020. "Yeah, we have to have border security", he said. There are about 700,000 young "Dreamers" who were brought into the USA illegally as children; advocates want to provide them with protection deportation and a chance to apply for citizenship.
Correspondents say Mr Castro does not have widespread name recognition and may struggle to make an impression in a crowded field.
His twin brother, Joaquin Castro, is a member of the U.S. House of Representatives from Texas. Kamala Harris and New Jersey Sen.More news: Steve Wilks interviewing for Browns defensive coordinator
He might not even be the frontrunner in his home state, which became immersed with Betomania in 2018 when Democratic Rep. Beto O'Rourke ran a solid campaign for U.S. Senator against Republican incumbent Ted Cruz.
Castro is aware he lacks the name recognition of potential 2020 rivals or the buzz surrounding O'Rourke, whose flirtations with 2020 have tantalized donors and activists after a close race previous year against Sen.
The former HUD secretary, who campaigned for O'Rourke in 2018, told the Associated Press last month he wasn't anxious if he himself is not testing very high right now in surveys. Diana Delrosario, a social worker in San Antonio, warned she might cry while she recounted how Castro once went out of his way as mayor to help wheel her mother out of a restaurant.
Castro, who has repeatedly dismissed talk that an O'Rourke candidacy would complicate his own chances, has framed the neighborhood and his upbringing as the story of an underdog.More news: Craig Brewer to Direct Coming To America 2, Eddie Murphy Returning
Mr Castro, whose grandmother was born in Mexico, has sought to use his family's personal story to criticize Mr Trump's border policies. It wasn't long before Democrats nationally embraced him as a star in the making, particularly one from Texas, where a booming Hispanic population is rapidly changing the state's demographics and improving the party's fortunes.
"I don't think it's necessarily about always having the shiniest words or approach, "Castro said".
Yet, in the years since his keynote speech in Charlotte, N.C., Castro has been content to remain on the sidelines as other Democrats moved ahead of him in attention and popularity.
Castro, who was considered on the short list to be Hillary Clinton's running mate in the 2016 election, has been out of the political spotlight for the past two years, at a time when politics has become all-consuming across America.More news: PC shipments drop again, just as things were starting to look up
Castro launched an exploratory committee for a potential campaign on December 12, citing a platform built on opportunity, education, housing, health care, immigration and the environment.