Failed Democratic presidential candidate Julian Castro is putting his political weight behind former rival Elizabeth Warren.

The endorsement from the only Hispanic candidate in 2020 could be a major boost for the U.S. senator for Massachusetts.

"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 pushed out by his former campaign team on Monday. "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."

Today I'm proud to endorse @ewarren for president.



Elizabeth and I share a vision of America where everyone counts. An America where people⁠—not the wealthy or well-connected⁠—are put first. I'm proud to join her in the fight for big, structural change. pic.twitter.com/xDvMEKqpF3 — Julián Castro (@JulianCastro) January 6, 2020

The former Obama administration housing secretary and mayor of San Antonio, Texas, 45, also announced he would be joining Warren, 70, at a rally in Brooklyn, New York, on Tuesday.

Castro, who ended his long shot White House bid last week, is only one of a few ousted presidential hopefuls to endorse someone else after suspending their campaigns. Ohio Rep. Tim Ryan, 46, publicly backed former Vice President Joe Biden, 77, in November, roughly a month after he exited the race.

With the Iowa caucuses looming, Castro's support comes at a crucial time for Warren's White House aspirations. The former Harvard Law School professor and special adviser to the Consumer Financial Protection Bureau has experienced flagging polling and fundraising numbers in the run-up to the Feb. 3 vote in the state. She trails Biden and Vermont Sen. Bernie Sanders in national polling averages with 14.4% of the vote, according to RealClearPolitics.

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