The last sentence of the Times’s article on the symbolically important, but practically not-so-important, endorsement from DC37:
“Another large union, Local 32BJ of the Service Employees International Union, which represents 60,000 doormen, janitors and other workers, is expected to endorse him in the next few weeks, several union officials said.”
Local 32BJ has quietly built itself into one of the four labor groups with a serious, large-scale political capacity, and brings with it at least one of the city’s top Democratic operatives, Peter Colavito. Mike has courted the big local, which represents janitors and doormen, assiduously. He embraced their plan to train private security guards, and this seems to be his reward. (The other key unions are 1199, the teachers, the trades, the last of which is already with Mike.)
Also, per that old Bloomberg-becomes-Mark-Green story: If you were on the street during the 2001 election, you’ll remember that 32BJ essentially was Mark Green’s field operation.