Julián Castro, the former housing secretary who was the only Latino candidate in the Democratic primary, said Thursday he would end his bid for the presidency, capping a yearlong campaign where despite struggling in polls, he remained an enduring contender and policy pacesetter on immigration and fighting poverty.

Throughout his campaign, Mr. Castro, 45, a native of San Antonio who spent five years as its mayor, portrayed himself as an unapologetic liberal who was shaped by his humble beginnings and had been overlooked by the press. Though he created some memorable moments as he championed progressive policy and challenged his rivals on the campaign trail, Mr. Castro was unable to break into the upper tier of a crowded primary field. His exit is the latest departure of a candidate of color from a field that began as the most racially diverse ever in a Democratic primary.

“I’ve determined that it simply isn’t our time,” Mr. Castro said in a nearly four-minute video message released by his campaign, which included a montage from his year on the trail, including visits to the border and a homeless encampment in Oakland. “Today it’s with a heavy heart, and profound gratitude, that I will suspend my campaign for president.”