The union, which represents some 35,000 workers, announced the endorsement at its biennial convention in Pittsburgh, where Sanders spoke Monday morning. It is the first endorsement Sanders has received from a national union this year.

The only other national union that has made an endorsement in the Democratic contest is the International Association of Fire Fighters, which backed former vice president Joe Biden shortly after he declared his candidacy. That union includes about 300,000 members in the United States and Canada.