When he won a special election on Feb. 20 in Bronx District 15 with 57 percent of the vote, Joel Rivera became the youngest member in the history of the City Council.

Mr. Rivera, 22, ran with the endorsement and active support of DC 37. The union’s Delegates voted overwhelmingly to back him on the recommendation of the Screening Committee, which meets with candidates and gets their positions on union-related issues.

Mr. Rivera will have to run again in November to keep the position, which opened up last November when his father, José Rivera was elected to the state Assembly.

DC 37 helped give Mr. Rivera the margin he needed to win. The union sent two mailings to members and launched a “people power” campaign that included volunteers at telephone banks and 41 activists in the district on Election Day.