Joaquin Castro says twin brother Julián Castro is running for president

In case there was any mystery left, Julián Castro is running for president.

That's according to his twin brother, U.S. Rep. Joaquin Castro, D-San Antonio, who made the announcement during a joint interview Thursday night on CBS's "The Late Show with Stephen Colbert." As Colbert teased Julián Castro about the build-up to his all-but-certain 2020 presidential bid, Joaquin Castro interjected.

"I'll speak on his behalf here," Joaquin Castro said. "He's gonna run for president. How about that?"

The comment drew a grin and nod from Julián Castro, the former U.S. housing secretary and San Antonio mayor.

The joint interview came the day after he announced that he was forming a presidential exploratory committee and would make an announcement Jan. 12 in San Antonio. For months, however, he has hardly concealed his ambitions, releasing a book, visiting early voting states and even saying in recent weeks that he is "very likely" to run.