WASHINGTON – Julian Castro, a former Obama Cabinet member and San Antonio mayor, officially threw his hat in the ring on Saturday, announcing he would run for president.

On a stage with his family, including his twin brother Rep. Joaquin Castro, in his hometown of San Antonio, the Texas Democrat declared his bid to try and take the White House from President Donald Trump in 2020 and fulfill a destiny that Democrats have projected since he was elected San Antonio mayor at 34.

"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 told a crowd of supporters in front of an American flag backdrop.

Castro, who made remarks in both English and Spanish, also rolled out his campaign slogan: One Nation. One Destiny.

Castro's campaign announcement at the historic Plaza Guadalupe came one day after Hawaii Rep. Tulsi Gabbard, 37, said she was also running for president, telling CNN "I have decided to run and will be making a formal announcement within the next week."

Castro, 44, along with Gabbard, have been predicted to be two of the youngest of expected candidates. Castro may also be the most well-known Latino figure in a race that's likely to revolve around immigration policy.

His announcement also followed Democratic U.S. Sen. Elizabeth Warren unveiling plans for an exploratory committee for president, a formality that comes before the launch of a presidential campaign that legally allows potential candidates to raise money.

On his way to the announcement, Castro rode a public bus to the plaza and live-streamed it to his Facebook page. The bus, No. 68, was the same one he and his twin brother rode to school as children.

His first trip as a candidate is scheduled for Puerto Rico, where he will visit with residents still coping with the effects of Hurricane Maria. He told supporters his first act as president, if elected, would be to rejoin the Paris Climate Accord, something that U.S. withdrew from under Trump. In his speech, Castro touched on many issues that are likely to guide the 2020 election season, including women's reproductive rights, Medicare for all, Black Lives Matter, climate change and immigration policy.

"Today we’re falling backwards instead of moving forward. And the opportunities that made America, the America we love, those opportunities are reaching fewer and fewer people," Castro told the crowd. "Today, we're at risk of dropping the baton. And that’s why we are all here today. Because we’re going to make sure that the promise of America is available to everyone."

Castro is likely to face stiff competition from a crowded field of high-profile Democrats in a primary challenge. Along with Gabbard and Warren, others, including former Vice President Joe Biden and New Jersey Sen. Cory Booker, are also said to be considering a run for the Democratic nomination.

But who seems to top the list of potential 2020 candidates is outgoing Rep. Beto O’Rourke, who lost in a November race against Sen. Ted Cruz, R-Texas. O’Rourke has excited donors and activists who have prodded him to seek the presidency.

Castro, who attended O’Rourke’s election-night party in El Paso, Texas, told the Associated Press in an interview last month that O’Rourke doesn’t complicate his own chances and downplayed figures that showed other prominent Democrats dwarf him in polls, saying in past election cycles early favorites ended up faltering.

More:Democrat Tulsi Gabbard announces 2020 run for president to take on Trump

More:Julian Castro moves to run for president in 2020, trails Beto badly in MoveOn wishlist

"People might say right now, 'Well, hey, you're way down here in polling that's taken.' The most dangerous place to be right now is actually in the pole position," Castro said. "It doesn't bother me that in December of 2018 I'm not right up at the top of the list."

Young and telegenic, Castro rose to national prominence early in his career as a Latino leader from a state that Democrats are eager to retake after decades of Republican dominance. But in Texas, O’Rourke has eclipsed Castro after getting closer to a statewide victory than any Democrat in a generation. It now puts Texas in the formerly unthinkable position of having two Democratic presidential candidates in the same year.

Obama picked Castro to take over the Department of Housing and Urban Development in 2014. Two years later, Castro was on the short list of Democratic presidential nominee Hillary Clinton’s potential running mates.

He quickly became a star in the party, marked by giving a keynote address at the Democratic National Convention in 2012.

But as Democrats have begun announcing their candidacy for 2020, the Republican National Committee has started their own campaign aimed at delegitimizing liberals. After Castro made his announcement, the Committee sent out a "cheat sheet" on Castro's record that characterized him as "weak" and used previous news coverage that described him as a "lightweight" and someone who didn't have enough experience for the office of the presidency.

"Julián Castro has made history by becoming one of the biggest lightweights to ever run for president," said Michael Ahrens, a spokesman for the Republican National Committee. "He was a weak mayor who couldn’t even handle being HUD secretary. This is obviously just another desperate attempt to become someone else’s running mate."

Since leaving the Obama Cabinet, Castro has established a political action committee called Opportunity First to help elect left-of-center candidates. In October, his memoir, "An Unlikely Journey: Waking Up from My American Dream," was released.

He is the grandson of a Mexican immigrant and son of a Latina activist.

Julian Castro has not run for office outside of San Antonio. But in a four-minute YouTube speech last month, he offered a preview of themes he might employ in what could be a long slog for the nomination.

"Growing up in Texas, America wasn't just my home, my country, it was also a promise," he said. "A promise that my family could have a better life. My grandmother, Victoria, came he when she was just 7 years old. She never made it past the fourth grade, but she taught her family the value of hard work."

The last Texas Democrat to run for president was Sen. Lloyd Bentsen, who had a short-lived campaign in 1976.

Maryland Rep. John Delaney, Richard Ojeda, who just lost a Congressional race in West Virginia, and Rep. Gabbard are the only declared 2020 Democratic presidential candidates so far. Others are expected to announce their intentions in the coming weeks.

Contributing: John C Moritz, The Associated Press