By Agence France-Presse
Castro would become the most high-profile Democrat to date to officially enter the 2020 race, which is expected to include a diverse field of candidates eager to challenge President Donald Trump.
But the 44-year-old would be among the underdogs in a political showdown that may well feature heavyweights like former vice president Joe Biden, US senators Elizabeth Warren, Bernie Sanders and Kamala Harris, and perhaps even billionaire businessman Michael Bloomberg.
Congresswoman Tulsi Gabbard of Hawaii, a progressive Democrat, said Friday she will officially announce her presidential run in the coming days.
Castro's strong oratory skills, experience as Barack Obama's housing secretary and as mayor of the nation's seventh largest city, not to mention his telegenic charisma, could help propel the Texas native into the top tier.
He would also become the third prominent Latino presidential candidate in four years, after Republican senators Ted Cruz and Marco Rubio fought Trump for their party's nomination in 2016.
Castro's candidacy could also help revive Democratic enthusiasm among Latino voters, who supported Hillary Clinton but at a lower rate than Obama.
Trump made immigration a flashpoint of that race, and it remains among the nation's most contentious issues. The US government remains locked in a partial shutdown, soon to begin its fourth week, over Trump's demand for $5.7 billion in border wall funding.
- A "great guy" -
Castro's national profile took off in 2012 when he became the first Latino to give a keynote address at the Democratic National Convention.
Four years later he was a finalist to be Clinton's running mate.
Castro, a fierce Trump critic, is a third generation American. Last year he said he probably would not be in the United States if today's strict immigration policies were in place when Castro's grandmother crossed the border as a young girl in 1922.
In his convention speech Castro highlighted "an unlikely journey" that he has lived, including growing up with twin brother Joaquin, who is a US congressman. The phrase became the title of his new memoir that was published last month.
He has kept the contents of his announcement secret, but he is widely expected to reveal his White House ambitions.
"I'll tell you and I'll tell the world what I'm going to do," he said this week on MSNBC.
Castro does not yet have the national recognition of luminaries Biden, Sanders and Warren.
Dena Grayson, a Democratic strategist, said Castro is a "great guy" and "phenomenal public speaker" who has potential broad appeal in states with large numbers of Hispanic voters, like California, Texas and Florida.
But standing out in a crowded field will be "difficult," Grayson added.