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.