SAN ANTONIO — Julián Castro, a former U.S. Cabinet secretary, launched his presidential campaign in the hometown where he served as mayor for three terms. The announcement surprised few, but puts him near the front of what is expected to be a jam-packed field of White House hopefuls.

Standing before a giant American flag at Plaza Guadalupe in the San Antonio West Side neighborhood where he grew up, Castro said in English and Spanish on Saturday that he is running for president.

Castro said his grandmother, who immigrated from Mexico nearly 100 years ago, never could have imagined that one of her grandsons —Castro's twin brother Joaquín — would be a member of Congress or that Castro himself would be standing on stage to say, "I am a candidate for the president of the United States of America." He then repeated that in Spanish.

Dozens of other Democrats who hope to challenge President Donald Trump in 2020 will likely follow fast after Castro. That field could include U.S. Rep. Beto O’Rourke of El Paso, Texas, who rose to national fame in a failed bid for the U.S. Senate and is scheduled next month for an interview with Oprah Winfrey.

That could be troublesome, in particular for Castro when it comes to raising money at least in Texas.

But the prominent Latino candidate's early-out-of-the-gate status gives the still-forming Democratic field an early appearance of racial diversity with a candidate whose ancestral roots are in Mexico — the country that Trump, in his 2015 presidential candidacy announcement, said was sending rapists, drug traffickers and criminals to the U.S.

In an interview with NBC News this week, Castro described himself as the “antidote” to Trump.

At the event, as an estimated 3,000 people cheered him on, Castro said, “You give me your support, and I give you my word: I will spend every day working hard to make sure you can get a good job, find a decent place to live, have good health care when you get sick and that your children and grandchildren can reach their dreams, no matter who you are or where you come from.”

Castro pledged that if elected, his first executive order would return the United States to the Paris Climate Accord, an international agreement on climate change. Trump withdrew the country from the pact. He also said his administration would invest in affordable housing for the middle class and poor.

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Castro made the announcement flanked by his wife Erica Lira Castro, his children Carina and Cristián. His mother Rosie, a Chicano activist who took him to political activities as a young boy introduced him as a son of San Antonio, son of Texas, a son of the West Side (of San Antonio), and a son of this country.

A mariachi band played as a crowd that had lined up around the block filed into the plaza. Castro rode to the event on a city bus, No. 68, which his campaign said was the same bus he and his brother took to school while broadcasting live on Facebook.

Joaquín Castro and other speakers made their case for his presidential run, underscoring a platform that contrasted Castro to Trump, with such things as supporting a free press and greater access to health care, his work creating educational opportunity and reducing homelessness for veterans.

Castro's first trip as a 2020 candidate is to Puerto Rico to speak at the Latino Victory Fund summit. One of the first items on Democratic legislators' agendas is calling for an investigation into the Trump administration's Hurricane Maria response.

Castro has a lot of work to do to introduce himself to Americans — or reminding them who he is.

Other potential candidates, such as Massachusetts Sen. Elizabeth Warren, who has formed an exploratory committee, Vermont Sen. Bernie Sanders, Sens. Kamala Harris of California and Cory Booker of New Jersey and O’Rourke, have had the benefit of the being in public office and the national spotlight recently.

Democratic Rep. Tulsi Gabbard of Hawaii announced Friday that she would also run for president.

But Castro's candidacy comes just after the party saw an increased Latino turnout in the 2018 midterms, helping the party to take control of the U.S. House.

"There is a real frustration in our community and desire to show we can’t be bullied,” said Oscar Ramirez, a Democratic strategist. “He has a real opportunity to show that our community has some real power and can’t be pushed around.”

Castro burst on the national scene in 2012 when he delivered the keynote address at the 2012 Democratic National Convention. But Hillary Clinton passed on naming him her running mate in 2016.

He was elected San Antonio’s mayor in 2009 and re-elected to two more terms before Obama tapped him to be secretary of HUD, the federal agency that oversees public housing, which Castro lived in during his youth.

At 26, Castro was the youngest person elected to the City Council of San Antonio, the nation’s seventh largest city.

Raised by his grandmother Victoriana and mother, Castro overcame his family’s modest upbringing to attend Stanford and Harvard Law School.

“He’s got an amazing story to tell,” said Democratic consultant Larry Gonzalez, a principal of the Raben Group in Washington. “The Castro family story is the quintessential American story. It is one that will resonate with a lot of Americans who are looking for opportunities that are slipping away. Julián will run a campaign that will address that and resonate with Americans.”

CORRECTION (Jan. 12, 2019, 4 p.m. ET): An earlier version of this article misstated to whom Julián Castro said his grandmother referred. He said she could have never imagined her grandson would be a member of Congress, not her son.