Former Housing Secretary Julián Castro announced he's endorsing Senator Elizabeth Warren for president, just days after he dropped his own bid for the Democratic nomination.

"Theres one candidate I see who's unafraid to fight like hell to make sure America's promise will be there for everyone," Castro said in a video released Monday. "That's why I'm proud to endorse Elizabeth Warren for president."

Castro dropped out of the presidential race last week after failing to break into the top tier of Democratic candidates. The Warren campaign said he will join the Massachusetts senator for a rally in Brooklyn on Tuesday to address supporters.

Castro will be an active surrogate for Warren who is planning to travel the country for her, according to people familiar with the plans. He will keep a handful of staffers from his campaign with him while others from his policy team are likely to join hers.

The endorsement forms a political marriage of like-minded campaigns and candidates. Castro and Warren are credited by the rest of the field for churning out some of the most detailed and liberal policy proposals, on issues like immigration, criminal justice reform and taxes. Castro often released his plans first and Warren followed with her own similar proposals, which inevitably garnered more attention.

In the video announcing his endorsement, Castro told Warren that voters "have confidence in you because you listened."

"More and more, they're recognizing that you're the one who's actually going to get it done," he said. 

The latest CBS News Battleground Tracker poll released Sunday showed Warren in fourth place in Iowa and third in New Hampshire, where her support has slipped since November.

The two showed a clear personal relationship and admiration for each other on the campaign trail. At Iowa's Polk County Steak Fry in September, Castro cut into her "selfie line" of thousands to say a brief hello. They embraced, posed for photos and spent a few minutes speaking, allowing a press scrum to gather and stoke speculation about whether they would eventually join forces. Neither side ever sought to tamper down that speculation.