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 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.

The endorsement is not a shock to close observers of the Democratic primary race — Mr. Castro and Ms. Warren made no secret of their shared affection for each other — but it formalizes a partnership that could help Ms. Warren reignite excitement at a critical moment.

Ms. Warren has fallen from her polling peak in early October, when she was hailed as the race’s ascendant front-runner and the standard-bearer for the party’s progressive wing. National polls now show Ms. Warren firmly in third, behind former Vice President Joseph R. Biden Jr. and Senator Bernie Sanders of Vermont, who has been aided by grass-roots progressive groups and by some high-profile endorsements of his own, including from popular House members like Representatives Alexandria Ocasio-Cortez of New York and Ilhan Omar of Minnesota.