Council Speaker Christine Quinn picked up the endorsement Tuesday of 32BJ, a big regional union that represents cleaning workers, doormen and security guards.
She also got the endorsement of a coalition of Teamsters leaders last week, only for Bill Thompson to peel off one of its member unions. Thompson also has the teachers, the city's largest public sector union.
Public Advocate Bill de Blasio has the endorsement of SEIU 1199, the city's largest union overall.
For 32BJ, the union said it came down to Quinn's deal-making ability on issues like paid sick leave, which she put up for a vote after years of advocacy by the union and others. And it made clear it wants to be remembered after Election Day.
32BJ endorsed Mayor Michael Bloomberg four years ago. He vetoed the paid sick leave bill, and the council is expected to override that veto this week.