Julián Castro endorsed Massachusetts U.S. Sen. Elizabeth Warren for president Monday, four days after ending his own bid for the White House.

"There's 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, "who will make sure that no matter where you live in America or where your family came from in the world, you have a path to opportunity, too. That's why I'm proud to endorse Elizabeth Warren for president."

Castro, the former U.S. housing secretary and San Antonio mayor, will join Warren for a joint rally Tuesday in Brooklyn, according to his campaign. Then he will head to Iowa this weekend to appear with Warren at a town hall Sunday in Marshalltown, her campaign said.

The endorsement is not entirely a surprise. Castro built a particularly friendly relationship with Warren while running, with the two trading compliments about their plans and sharing embraces on the campaign trail.

In the video, which is nearly three and a half minutes, Castro appears to visit Warren at her home, and the two swap praise for one another while seated at her kitchen counter.

Castro ended his campaign Thursday after struggling for months to gain the traction needed to be a top-tier candidate, despite often being the first in the field to champion certain progressive causes. In the video, Warren tells Castro, "You did so many things in this campaign, and it continues to matter."

Castro could be helpful for Warren in Texas, where she has been the most proactive candidate in building a formal organization ahead of the state's March 3 primary. In an MSNBC interview Tuesday night, Warren suggested Castro would be a major surrogate for her going forward, though she brushed off speculation that he could end up as her running mate.

"That would be presumptuous to talk about," Warren said, "but Julián is a great partner in these fights, and I'm glad to be there with him."