Updated Jan. 2 at 9:15 p.m. with more details on Castro's Jan. 12 event. Story was originally published Dec. 12, 2018.

WASHINGTON — Plans are coming into focus for Julián Castro's presidential campaign rollout at an outdoor rally Jan. 12 in San Antonio.

The budding campaign told supporters by email Wednesday night that the event -- billed as a "special announcement about his 2020 presidential plans" -- will be at the Plaza Guadalupe, an outdoor amphitheater just west of downtown.

The formal campaign kickoff will come exactly one month after Castro formed a presidential exploratory committee, giving the former Obama housing chief and San Antonio mayor a head start in a crowded field eager to challenge President Donald Trump.

Only a day later, his twin brother, U.S. Rep. Joaquin Castro, told Stephen Colbert during a joint appearance on the Late Show that there is no mystery: Julián will definitely run.

"Americans are ready to climb out of this darkness," Castro says in a four-minute video laying out his likely candidacy.

Nearly two dozen Democrats are eyeing a 2020 run, and there's no clear front-runner. Sen. Elizabeth Warren of Massachusetts formed an exploratory committee on Dec. 31, and a number of lesser-known Democrats have been stumping for months.

Castro's event would be the first formal campaign kickoff in the 2020 field. The plaza is a common site for campaign events. El Paso's Beto O'Rourke drew hundreds of supporters on a rainy afternoon three weeks before Election Night.

O'Rourke's six years in the U.S. House officially end at noon Thursday, and if he runs, as many Democrats around the country are urging, the field would have two Texans — a remarkable turn of events for a state that hasn't elected a Democrat statewide since 1994.

O'Rourke largely eclipsed Castro with his near-miss effort to unseat Sen. Ted Cruz. That effort drew national buzz, huge crowds and an eye-popping $80 million in donations.

Castro has not sought statewide office. He had been laying the groundwork for a presidential bid for months, only to see O'Rourke shoot past him and into the top tier in surveys of Democratic voters, though name recognition weighs heavily in early presidential polling.

Since O'Rourke held Cruz below 51 percent, party donors and activists have prodded him to seek the presidency. He has been talking to party leaders in recent weeks, including Obama, while fielding invitations to appear in Iowa and New Hampshire.

Speaking outside the Capitol last month, O'Rourke offered nothing but good wishes for Castro.

"I'm happy for Texas, I'm happy for the country that he's in the race," he said. "I've just gotten to know him over the last few years and just think he's a great person and will make a great candidate, and if he wins, he will make a great president."

Some Democrats are skeptical about conferring the nomination on someone whose last election bid ended in defeat as O'Rourke's did.

And Castro, 44, has executive experience that his fellow Texan lacks, running a city and a Cabinet department. He's also much further along at reaching out to activists in the states with the early contests that winnow the field.

By Election Day last month, Castro had already visited New Hampshire three times this year. He made his first trip to Iowa in August, meeting with party activists, speaking at the Iowa State Fair, insisting that pundits who've begun laying odds on potential contenders such as himself are "getting it wrong," and saying he'll raise all the money he needs "when the time comes, if I decide to run."

In Storm Lake, Iowa, a meat packing center where half of the populace is Latino, he addressed voters in Spanish.

Castro's twin brother, Rep. Joaquin Castro of San Antonio, newly elected chairman of the Congressional Hispanic Caucus, is likely to have a central role in the campaign effort.

"I wholeheartedly support my brother and will do everything I can for him," he said Wednesday outside the Capitol. "I'll do whatever he asks me to do. I expect to have a really active role on the campaign."

As for the O'Rourke factor, he said, "My brother, I think, is resolved to run. Beto is a great guy and has been a good friend over the years. We wish him well in his decision. We were glad to support him in his Senate race in 2018."

The brothers appeared last month on The Late Show with Stephen Colbert.

Thursday night, my brother @JulianCastro and I will be on the @colbertlateshow! Tune in at 10:35pm CT! pic.twitter.com/Z93it4lQu7 — Joaquin Castro (@Castro4Congress) December 10, 2018

In a roll-out video Dec. 12, Julián Castro makes no mention of any rivals and only a passing reference to Trump. He focuses on his family's story, presenting a vision of an America that welcomes immigrants, in contrast to the current administration's approach.

"We have room for you," he says.

As a kid growing up on the west side of San Antonio, I never thought that I’d one day be making this announcement: https://t.co/2NAIFEsCFh I’m exploring a candidacy for President of the United States in 2020 to renew the promise of this country for all. https://t.co/9jOBdjHcLO — Julián Castro (@JulianCastro) December 12, 2018

"This is a place where dreams can become real. No matter where we're from, we're united by the same daily needs: a good job, a good education for our kids, good health care, an affordable place to live. health care. The need to be acknowledged for our contributions, not for our gender or for who we love," Castro says in the video. "We all hope our children have clean air to breathe and clean water to drink.

"I've spent the last several years traveling around America and listening to people's concerns. Mothers from Arizona, college students in Iowa, teachers in Florida," he says.

Taking a cue from O'Rourke, Castro told reporters at his home in San Antonio that he would reject donations from political action committees.

.@JulianCastro: “I’m not gonna take PAC money if I run.” — Patrick Svitek (@PatrickSvitek) December 12, 2018

O'Rourke's decision

O'Rourke said last month that he wouldn't make a decision before his term in Congress ends, and his timing isn't affected by moves by Castro or anyone else because "we're so far from making any kind of decision or even thinking about any kind of decision."

"He was a terrific mayor in San Antonio," O'Rourke said, lauding, among Castro's other achievements, a program called Pre-K 4 SA. "Some form of that should be implemented nationally — universal guaranteed pre-kindergarten education for every child in America."

An exploratory committee usually is a formality before formally launching a campaign. It allows potential candidates to raise and spend money while delaying the scrutiny that comes once they formally enter the race.

Few people I know have lived up to the promise of the American dream and Texas’ legacy of bold leadership more than Sec. @JulianCastro. I am honored to call him a friend and a mentor, and like many Texans, I enthusiastically welcome his consideration of a presidential campaign. pic.twitter.com/EoSvejhJSM — Colin Allred (@ColinAllredTX) December 12, 2018

Colin Allred, the Dallas Democrat who ousted longtime congressman Pete Sessions, offered high praise but left room to say he hadn't issued an exclusive endorsement.

"I am honored to call him a friend and a mentor, and like many Texans, I enthusiastically welcome his consideration of a presidential campaign," Allred said in a statement. "As I know first hand from my work for Secretary Castro at the U.S. Department of Housing and Urban Development, his style of leadership, temperament and experience is exactly what we need right now in the White House."

Other Texas Democrats, including O'Rourke's successor in Congress, Veronica Escobar, also praised Castro while stopping short of an endorsement.

Thank you for stepping up for our country, @JulianCastro. What we need more than ever - because of the cruelty and ineptitude of the last (nearly) 3 years - is ethical, principled, visionary leaders and candidates bringing forward the best ideas and values for America. 🇺🇸 https://t.co/NvBwR0IGag — Veronica Escobar (@vgescobar) December 12, 2018

Early jump on competitors

Castro's move gives him a jump on bigger-name Democrats who are considering a run but are taking a slower approach.

Sen. Cory Booker of New Jersey and former Vice President Joe Biden are also potential candidates.

Castro would be among the youngest in the field and the most prominent Latino, a community he said has been treated "like a pinata" under Trump.

Castro stumped for O'Rourke and attended his election-night party in El Paso last month, and said his fellow Texan doesn't complicate his own chances.

"He's talented. He ran a good race against Ted Cruz," Castro told the AP. "I'll let him talk about his future."

Castro has not spoken about his plans with Obama, who picked him take over the Department of Housing and Urban Development in 2014. Two years later, Castro was on Hillary Clinton's short list for vice president.

Democrats have eyed Castro since he was elected San Antonio mayor at 34, followed by his star-making turn as the keynote speaker at the Democratic National Convention in 2012.

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

His brother already had a high profile in Congress, and as head of the Congressional Hispanic Caucus, he will be at the center of debates over immigration and border security during the second half of Trump's term.

The last Texas Democrat to run for president was Sen. Lloyd Bentsen, who had a short-lived campaign in 1976. Bentsen ended up on the 1988 ticket as Michael Dukakis' running mate, and served as Treasury secretary under Bill Clinton.

Maryland Rep. John Delaney is the only declared 2020 Democratic presidential candidate so far. Others are expected to announce their intentions in the coming weeks.

This report includes material from The Associated Press.