At a rally in San Antonio's Plaza Guadalupe, Castro said, "There are no front-runners that are born here, but I've always believed that with big dreams and hard work, anything is possible in this country."
"I'm running for president because it's time for new leadership, new energy and it's time for a new commitment to make sure that the opportunities I've had are available to every American," he added.
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Castro said he aimed to make the United States "the smartest, the healthiest, the fairest and the most prosperous nation in the world." He said that his plans will include expanding Medicare to allow access for all and addressing what he called a housing affordability crisis in which rising rents are squeezing the poor and middle class. Castro added that he will not accept campaign donations from political action committees.
VIDEO: Castro on possible Beto O'Rouke 2020 campaign
Castro comments on O'Rourke's chances of running
Since announcing his exploratory committee in December, Castro has visited Iowa and Nevada. He will visit New Hampshire this week.
The former San Antonio mayor joins Hawaii Rep. Tulsi Gabbard, Massachusetts Sen. Elizabeth Warren and former Maryland Rep. John Delaney, just some of the candidates who have formed exploratory committees or announced intentions to run in 2020.
Castro joined San Antonio's city council as its youngest member in 2001 and was elected mayor eight years later. As mayor, Castro became well known for his efforts to promote development within the city.
SEE ALSO: What to know about Julián Castro
Julian Castro: What to know about former HUD secretary, San Antonio mayor
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A rising star within the Democratic party in the early 2010s, Castro became the first Hispanic person to give a keynote address at the Democratic National Convention in 2012.
He joined President Barack Obama's administration in 2014 as the secretary of the U.S. Department of Housing and Urban Development.
Castro endorsed Hillary Clinton in 2016 and was at one point viewed as a possible running mate for the former secretary of state.