Former Democratic U.S. presidential candidate Julian Castro waves at a campaign event held by Democratic U.S. presidential candidate and Senator Elizabeth Warren at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Former Democratic U.S. presidential candidate Julian Castro reacts as he speaks at a campaign event held by Democratic U.S. presidential candidate and Senator Elizabeth Warren at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Former Democratic U.S. presidential candidate Julian Castro speaks at a campaign event held by Democratic U.S. presidential candidate and Senator Elizabeth Warren at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Former Democratic U.S. presidential candidate Julian Castro and Democratic U.S. presidential candidate and Senator Elizabeth Warren react before embracing during a campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren embraces former Democratic U.S. presidential candidate Julian Castro during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren gestures next to former Democratic U.S. presidential candidate Julian Castro during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Former Democratic U.S. presidential candidate Julian Castro and Democratic U.S. presidential candidate and Senator Elizabeth Warren wave to the audience after embracing during a campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren reacts between former Democratic U.S. presidential candidate Julian Castro and supporters during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren acknowledges the audience during a campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren reacts during a campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren looks on during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren gestures to the audience as former Democratic U.S. presidential candidate Julian Castro looks on during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Supporters listen as Democratic U.S. presidential candidate and Senator Elizabeth Warren speaks at her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren speaks as former Democratic U.S. presidential candidate Julian Castro looks on during her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri

Democratic U.S. presidential candidate and Senator Elizabeth Warren and former Democratic U.S. presidential candidate Julian Castro attend her campaign event at Brooklyn's Kings Theatre in New York, January 7, 2020. REUTERS/Carlo Allegri