Now the 37-year-old has a chance to shine in prime time Tuesday as the first Latino to deliver the keynote address at his party's national convention.
Castro has hinted that his speech will focus heavily on defending President Barack Obama's record while also telling his story of growing up poor with his identical twin brother, Joaquin.
Joaquin is a Texas state legislator now representing San Antonio and poised to win election to Congress in November. He will introduce Julian at the convention opener Tuesday night.
