San Antonio (CNN) Former secretary of Housing and Urban Development Julián Castro officially announced his presidential bid in San Antonio on Saturday, beginning a campaign that will look to turn his uniquely American immigrant story into a direct repudiation of President Donald Trump.

"When my grandmother got here almost a hundred years ago, I'm sure she never could have imagined that just two generations later, one of her grandsons would be serving as a member of the United States Congress and the other would be standing with you here today to say these words: I am a candidate for President of the United States of America," Castro said.

Castro, the former mayor of San Antonio, had been considering a bid for nearly two years and announced a presidential exploratory committee in December. He has long been viewed as a rising star in the Democratic Party since he first landed on the national scene by delivering the keynote speech for President Barack Obama at the 2012 Democratic National Convention.

Castro, in an interview with CNN ahead of his official presidential announcement, acknowledged that he will enter the race looking up at the cadre of other Democrats considering running for the party's nomination.

But Castro, whose grandmother, Victoria Castro, was born in the Mexican border state of Coahuila, and crossed into the United States at Eagle Pass, Texas, in 1922 after her parents died during the Mexican Revolution, added that at no time in his life, from growing up on San Antonio's impoverished West Side to his run for mayor in 2009, was he considered the favorite to get ahead.

"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. "I am going to go speak to them in a way that resonates with them."

He added: "My family's story is a testament to what is possible when this country gets it right."

Castro's personal story, along with that of his twin brother, Joaquin, has been central to his rise on the national stage and made up the bulk of his 2012 convention speech.

Castro's brother - who also serves as a member of Congress - will serve as his campaign chairman, according to a campaign press release provided to CNN.

Castro was raised primarily by his grandmother -- who he called Mamo -- and Rosie Castro, his Chicana political activist mother, eventually excelling enough to attend Stanford University and, eventually, Harvard Law School. He returned home and served as a member of the San Antonio City Council and, from 2009 until 2014, the mayor of the city. Obama picked him to be housing secretary in 2014.

The former mayor was among a handful of contenders to be Clinton's vice presidential pick in 2016. Although he was eventually passed over for Sen. Tim Kaine of Virginia, Castro said Clinton's loss and Trump's victory was the moment he began considering a run.

"Donald Trump represents the opposite of what I am and what I believe," he said. "For many Americans, a lot changed when Donald Trump got into office. And that is what has compelled me to think about running."

Castro slammed Trump throughout his announcement speech, particularly some of the language he uses to describe immigrants and his plan to build a wall along the US-Mexico border.

"We say no to building a wall and say yes to building community. We say no to scapegoating immigrants, and yes to Dreamers, yes to keeping families together, and yes to finally passing comprehensive immigration reform," Castro said to applause.

Castro, in an interview with CNN after his announcement, dismissed the suggestion that former Texas Rep. Beto O'Rourke running for the Democratic nomination against him would damage his chances, but he did admit that he wishes the popular Democrat would stay on the sidelines.

"Of course, I'd rather be the only Democrat running from Texas," Castro said. "But I think that we are going to have a crowded primary, we don't know who is going to run and so everybody is going to have to go and put out their vision and do the hard work of campaigning and reaching voters."

Castro called himself the "antidote to Trump" because "my story is am immigrant story, is a testament to what immigrants have contributed to this country."

Castro's entrance makes him the only declared Latino in the Democratic field, a relatively powerful position given how the party has leaned on Latino voters and turnout efforts to tilt states like Nevada, Arizona and Texas their direction in recent years.

But Castro told CNN that he believes he can perform well in Iowa and is backing that confidence up, according to a source with knowledge of Castro's operation, by naming Derek Eadon, the former chairman of the Iowa Democratic Party, to a senior role on his nascent campaign.

Maya Rupert, formerly the executive director of Castro's PAC, will serve as campaign manager and Derek Eadon, formerly the chairman of the Iowa Democratic Party, will be Castro's deputy campaign manager.

Castro and his team have signaled in the lead up to Saturday's announcement that he will lean into his Mexican-American heritage in a presidential run. When the former mayor filed paperwork with the Federal Election Commission, he had to hand write in an accent over "a" in Julián, a fact that Castro and his team have held up as proof that he is already changing the system.

Evidence of this strategy peppered the plaza on San Antonio's West Side where Castro announced on Saturday: A mariachi band welcomed guests, as taco trucks fed people outside. The venue was blocks from Castro's childhood home and across the street from where he was baptized. Even Castro's logo - which emphasizes the accent over the "a" in Julian - highlights the former mayor's heritage.

The audience in San Antonio was filled with people who knew Castro from his youth, and people who were impacted by his time in elected office.

Alma Palacios, a Mexican immigrant, decided to name her son Julián after the city's former mayor and now presidential candidate. And the younger Julián, sporting a full Spider-Man outfit, watched as Castro announced his bid.

"His is an inspiration and a good role model to all of us Hispanics," Palacios said. "It means a lot (to see him announce)."

Castro does not speak fluent Spanish, writing in his 2018 memoir that his mother spoke English at home, like many immigrants at the time, and that he declined to take Spanish classes in school because he spoke it with his grandmother.

"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 delivered the crescendo of his speech in Spanish and closed with a similar call to action.

"So, let's get to work," he bellowed. "Vamonos!"

Castro will travel to San Juan, Puerto Rico, for a campaign event on Monday, before heading to New Hampshire on Tuesday.

Castro said ahead of his run that he hopes to provide a "positive example" to young Latinos with his run but won't solely focus on courting voters in heavily Latino states and shirk visiting states like Iowa, New Hampshire and South Carolina.

"I am just going to be myself," he said. "My focus will be about representing everybody but of course there is a special significance to the Latino community especially because many Latinos feels like there is a target on their back from this administration."

His announcement speech also previewed a candidacy that will focus on good government, education and climate change, an issue Castro called "the biggest threat to our prosperity in the 21st century."

The Republican National Committee slammed Castro's announcement, calling him "another delusional Democrat."

Castro was reflective days before his announcement, acknowledging how his wife's support and mother's political work helped him get to this point. If he had a disappointment, he said, it was that is grandmother, who died when he was younger, could not be there to see him.

"I wish my grandmother could be with us to see it," he said. "Win or lose, I hope that I will have a chance to inspire a lot of young kids out there to reach for their dreams."