In his convention speech Castro highlighted "an unlikely journey" that he has lived, including growing up with twin brother Joaquin, who is a U.S. congressman.
"I am not a frontrunner in this race, but I have not been a frontrunner at any time in my life", Castro said, adding that people who grew up in the neighborhoods he grew up in were never considered frontrunners.
Still, with progressive Democrats urging that the party try something different in its rematch against the Republicans and President Donald Trump, Castro - with a few breaks and hard work - could make a strong run.
He made his announcement at Plaza Guadalupe, a landmark in San Antonio's sprawling west side Mexican-American barrio, a neighbourhood of neat, brightly painted wood framed homes, many on tiny lots and festooned with white plaster Catholic statues.
Castro, 44, also served as the mayor of San Antonio, Texas from 2009 until he joined Obama's cabinet in 2014.
Castro was reflective days before his announcement, acknowledging how his wife's support and mother's political work helped him get to this point. John Delaney, a former congressman from Maryland, launched his presidential bid in 2017, and Sen. Cory Booker - among many other - vying for the White House.
At a time when the federal government has been partly shut down over Trump's demand for funds to build a wall on the Mexican border, Castro sounded a contrasting message.
In an interview with NBC News on Friday, Castro said: "Mine is an immigrant story".
"I've resolved that before I die, I want to speak it fluently", said Castro, who has used the program Rosetta Stone to learn the language.
Castro would be the first identical twin elected president. "And there is no way in hell that caging children is keeping us safe", Castro said.
So, too, is former Rep. Beto O'Rourke, D-Texas., who could be more of a challenge.
Former Rep. Beto O'Rourke, who narrowly lost a Senate bid past year, is also weighing a run and has been on the rise in very early polls. And more importantly, for 2020's sake, O'Rourke became a national name and one of the frontrunners for all Democrats - at least 30 are expected to toss their names in the hat.
Castro may not be the only Texan in the race, however. He was the youngest-ever city councilman in San Antonio's history when he was elected in 2001 at age 26.
National Democrats, looking to develop young Hispanic leaders and make inroads in that critical election demographic, began mentioning Castro as a budding star.
The 44-year-old, who rose to national prominence with an electrifying keynote speech at the 2012 Democratic convention, seems set to become the second high-profile Democrat to announce their candidacy for 2020 following MA senator Elizabeth Warren's campaign launch at the end of last year.
Castro was vetted as a potential vice-presidential running mate to Hillary Clinton in 2016, but was overlooked as Virginia senator Tim Kaine was given the role.
"Julian Castro shouldn't be counted out completely", Jones said.
Like other Democrats running, Mr Castro has said he will not accept money from political action committees tied to corporations and unions, and he has sought to introduce himself to voters as a champion for universal health care and affordable housing.
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