Castro, 44, the grandson of a Mexican immigrant who would be the first Hispanic elected president, served as secretary of Housing and Urban Development under former President Barack Obama and has long been viewed as a rising star in the party.
Former mayor of San Antonio, Texas and Obama-era cabinet member Julian Castro on Saturday launched his bid to become the nation's first Hispanic president, making his announcement in a border state central to America's immigration debate.
"I am a candidate for president of the United States," the 44-year-old Castro told a large crowd in San Antonio's historic Guadalupe Plaza, in a speech that frequently invoked his immigrant heritage.
Often called a rising star in the Democratic Party, Castro, who was Obama's secretary of housing, is expected to be part of a huge and diverse field of candidates eager to challenge President Donald Trump.
Urging the enthusiastic crowd to look around the blue-collar neighbourhood where he grew up, Castro said, "There are no front-runners that are born here, but I've always believed that with big dreams and hard work, anything is possible in this country."
He said he and twin brother Joaquin, who is a member of Congress, rode to Saturday's event on the same bus line that once took them to public school.
A rising star in the Democratic Party
Castro added that his grandmother Victoria, who sat on a stage behind him beneath a huge American flag, would surely have been amazed when she arrived from Mexico in 1922, a seven-year-old orphan, had she known that one grandchild would end up in Congress and the other as a presidential candidate.
At a time when the federal government has been partly shut down as Trump demands funds to build a wall on the Mexican border, Castro sounded a contrasting message.
San Antonio, he said, "represents America's future: diverse, fast-growing, optimistic, a place where people of different backgrounds can come together to create something truly special."
Castro's strong oratory skills, experience in the Obama cabinet 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 of Democratic candidates.
But he would start out as one of 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.