SAN ANTONIO/WASHINGTON • Former mayor of San Antonio, Texas, and Obama-era Cabinet member Julian Castro was expected yesterday to launch his bid to become the US' first Hispanic president, scheduling his announcement in a border state central to the country's immigration debate.

Mr Castro, 44, would become the most high-profile Democrat to date to officially enter the 2020 race, which is expected to see a diverse field of candidates eager to challenge President Donald Trump.

But he would be among the underdogs in a political showdown that may well feature heavyweights such as former vice-president Joe Biden, senators Elizabeth Warren, Bernie Sanders and Kamala Harris, and perhaps even billionaire businessman Michael Bloomberg.

Also in the race would be Congressman Tulsi Gabbard of Hawaii, a progressive Democrat, who is to officially announce her run soon.

Mr Castro's strong oratory skills, experience as Mr Barack Obama's housing secretary and as mayor of the US' seventh-largest city, not to mention his telegenic charisma, could help propel the Texas native into the top tier.

He would be the third prominent Latino presidential candidate in four years, after Republican senators Ted Cruz and Marco Rubio fought Mr Trump for their party's nomination in 2016.

Mr Castro's candidacy could also help revive Democratic enthusiasm among Latino voters, who supported Mrs Hillary Clinton but at a lower rate than for Mr Obama.

Mr Trump made immigration a flashpoint of that race, and it is still among the most contentious issues in the US. The government is in a partial shutdown, soon to begin its fourth week, over Mr Trump's demand for US$5.7 billion (S$7.7 billion) in border wall funding.

Mr 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 Mrs Clinton's running mate.

A fierce Trump critic, Mr Castro 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 his grandmother crossed the border as a girl in 1922.

In his convention speech, Mr Castro highlighted "an unlikely journey", including growing up with his twin brother Joaquin, who is a US congressman. The phrase became the title of his new memoir that was published last month.

Meanwhile, Ms Gabbard, an Iraq War veteran who met dictator Bashar al-Assad in Syria during its devastating civil war, told CNN last Friday: "I have decided to run and will be making a formal announcement within the next week."

If elected, the 37-year-old lawmaker would be the youngest president in US history. She is the first Hindu member of Congress and also its first Samoan American.

"There are a lot of reasons for me to make this decision. There are a lot of challenges that are facing the American people that I'm concerned about and that I want to help solve," she said, citing healthcare access and climate change as among the issues.

Ms Gabbard sits on the House Foreign Affairs Committee, where she is able to influence US foreign policy. "There is one main issue that is central to the rest, and that is the issue of war and peace," she said.

She was deployed to Iraq in 2005, and she maintains her role as a member of the Hawaii National Guard.

AGENCE FRANCE-PRESSE