Julián Castro makes his support for Joe Biden official

Julián Castro formally backed Joe Biden for president Tuesday after the presumptive Democratic nominee's call for police reform in the wake of George Floyd's death.

Castro, the former Biden primary opponent, U.S. housing secretary and San Antonio mayor, said in a tweet he was "proud to support" Biden, and a Castro spokesman confirmed he was endorsing the former vice president. While Castro had called for unity around Biden and vowed to help defeat President Donald Trump, he had not formalized his support for Biden until now.

Castro's tweet came shortly after Biden delivered a speech in which he called on Congress "to act this month on ... real police reform" after the death of Floyd, the black Minnesota man who died after a white police officer spent almost nine minutes kneeling on his neck. Castro championed police reform during his own presidential campaign last year, releasing a platform on the issue and frequently listing the names of police violence victims on the stump.

"Joe Biden recognizes the urgent need for real reform to address our broken policing system," Castro tweeted, adding that he looks "forward to seeing these reforms become law, so that what happened to George Floyd never happens again."

Biden responded to Castro's endorsement a short time later, promising to work with his former rival on fixing policing.

"We're going to tackle this head on — and we're going to need your help to do it," Biden tweeted at Castro. "Grateful for your support."

Castro's lack of a formal Biden endorsement had drawn some increasing attention in recent weeks. His brother, U.S. Rep. Joaquin Castro of San Antonio, has not officially backed Biden, though a Joaquin Castro spokesman said in a statement last week that the congressman is "going to do everything he can to make sure Joe Biden beats Trump like a drum."

Both Castros endorsed Elizabeth Warren after Julián Castro dropped his bid at the start of the year. She ended her run in early March and endorsed Biden the next month.