Castro's identical twin brother, Rep. Joaquín Castro (D-Texas), will serve as campaign chairman, Castro's campaign said.
As Fox News noted, Castro is running on a progressive platform that includes universal health care and affordable housing. Gabbard, the first American Samoan and the first Hindu member of Congress, is a veteran of the Iraq war and at 37, is another young candidate.
Democratic former USA housing secretary Julian Castro is widely expected to launch a 2020 presidential bid.
Making the announcement at Plaza Guadalupe in the San Antonio's West Side neighborhood where he grew up, Castro said in English and Spanish that he is running for president.
Castro, who could end up being the only Latino in what is shaping up to be a crowded Democratic field, officially kicked off his campaign with a rally in his hometown of San Antonio, where he was mayor for five years.
Castro was reflective days before his announcement, acknowledging how his wife's support and mother's political work helped him get to this point.
Until now only former congressman John Delaney has formally launched a campaign, more than a year ago.
Should Castro win the Democratic nomination and eventual presidency, he would become one of the youngest Americans to ever hold the White House, and he would be the first Hispanic to do so.
Wisconsin bus driver rescues child
When authorities arrived at the scene, the baby boy was fast asleep in Ivic's arms and the passenger's coat. Ivic stopped the bus and ran out to scoop up the baby, as seen in a video released by the transit system.
In an interview with NBC News on Friday, Castro said: "Mine is an immigrant story". 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.
The presidential candidacy is Castro's first attempt for office beyond Texas' second-most populous city.
Castro, whose grandmother came to the United States from Mexico in the 1920s, would be vying to become the party's first Hispanic-American nominee amid a field of that could swell to more than 20 candidates.
Among the other possible candidates expected to make decisions soon are Senators Cory Booker of New Jersey and Kamala Harris of California. Ted Cruz turned O'Rourke into a national Democratic celebrity.
He said San Antonio, a city that is almost two thirds Hispanic, "represents America's future: diverse, fast-growing, optimistic". In 2012, he gave the keynote address at the Democratic National Convention - which had catapulted Barack Obama to national fame eight years earlier - telling the crowd about his experience as part of an immigrant family.
In addition, Castro is a longtime supporter of LGBTQ rights, Black Lives Matter, and has strongly condemned Trump's immigration policies, especially separating families at the border.
Two years later, Mr Obama picked him to lead the Department of Housing and Urban Development. He was also famously considered a frontrunner to be former Secretary of State Hillary Clinton's running mate in 2016, but she ultimately went with Senator Tim Kaine.
"I know that the press work hard and that they are the friend of the truth in this country", Castro told the media at his event.
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.
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