He was projected to win the San Antonio seat early Tuesday night. CNN had Castro ahead of Republican David Rosa 61-36 with just 8 percent of polls reporting.

During the Democratic convention, Julián Castro called Mitt Romney the candidate of "no," and charged the GOP presidential nominee with flip-flopping on healthcare for supporting health mandates while Massachusetts governor.

"Governor Romney has undergone an extreme makeover, and it ain't pretty," he said.