Julián Castro, the former housing secretary whose progressive presidential candidacy did not make significant inroads with Democratic voters but earned plaudits from the party’s left wing, has endorsed Senator Elizabeth Warren of Massachusetts, offering a possible lift for her candidacy less than one month before the Iowa caucuses.

Mr. Castro announced his endorsement on Monday morning, just days after he ended his own bid for the White House. In a statement, Mr. Castro cast Ms. Warren as the logical extension of his campaign’s social-justice-driven message, which seeks to correct inequities through targeted policy proposals. He will campaign with Ms. Warren this week, joining her Tuesday night at a rally at Kings Theatre in Brooklyn.