Castro, who also served as secretary of housing and urban development under President Barack Obama, will join Warren at a town hall in Brooklyn on Tuesday.
During his presidential run, Castro built a reputation for speaking out early on a range of issues and for targeting minority groups that drew less attention from other campaigns.
Monday’s endorsement, just days after Castro departed the race, is in keeping with the lack of hesitation he demonstrated during his campaign: Other than Rep. Tim Ryan (D-Ohio), who endorsed former vice president Joe Biden a few weeks after ending his campaign, none of the major players in this year’s Democratic race have announced their endorsements.
High-profile former candidates such as Sen. Kamala D. Harris (D-Calif.), Sen. Kirsten Gillibrand (D-N.Y.), Rep. Beto O’Rourke (D-Tex.) and others have not yet offered their support to former rivals.
Castro’s endorsement, coming weeks before the Iowa caucuses, could offer Warren’s campaign a timely boost. Recent polls in Iowa and New Hampshire show her falling behind Biden, Sen. Bernie Sanders (I-Vt.) and former South Bend, Ind., mayor Pete Buttigieg in those early states. Warren raised $21.2 million in the fourth quarter of 2019, less than Sanders, Buttigieg or Biden.
Warren has also failed to amass the significant team of surrogates accumulated by others. Sanders has hosted rallies with members of the “squad” of liberal congresswomen, as well as trotting out actors and musicians and others. Biden is sending former secretary of state John F. Kerry and Atlanta Mayor Keisha Bottoms to Iowa this week, and he recently nabbed the endorsement of Iowa Rep. Abby Finkenauer, among others.
When Castro dropped out of the race last week, he said he had “determined that it simply isn’t our time.” He had failed to qualify for the past two Democratic debates because he had not met the party’s thresholds. Despite his struggles in the campaign, Castro could help Warren with liberal Democrats, among whom he is well-liked.
Throughout his campaign, Castro cultivated a reputation as a liberal Democrat who focused his attention on the poor and other marginalized communities. His first trip after announcing his candidacy was to Puerto Rico, and he notably called for decriminalizing the act of crossing the border without legal permission.
In the weeks before he dropped out, Castro grew more vocal about why Iowa voters should not go first in the primary process, arguing that the state — whose population is 90 percent white — is not representative of the country’s diversity.
“I’m not interested in changing the rules of the game in the middle of the game,” Castro said at a December town hall in Des Moines. “What I am interested in is changing for the future.”