From NY Magazine:
The mayor is not a dictator. He can’t unilaterally decree major changes, and he still has to strike bargains with two legislative bodies—the City Council (generally a formality) and the State Legislature (more difficult, particularly when dealing with the inscrutable Sheldon Silver). But Bloomberg gets what he wants more than any mayor in modern memory.
The foundation of Bloomberg’s imperial mayoralty is, obviously, money. He’s used his vast personal fortune—$17.5 billion at last Forbes estimate—relentlessly and creatively to reverse the standard political dynamic: Instead of the special interests’ buying off the politicians, the city’s top politician has bought off the special interests. Money has allowed him to create the Bloomberg Party, whose clubhouse is the business elite and whose field troops are enlisted issue by issue. Bloomberg employs large segments of the city’s political class, directly and indirectly, and his philanthropy, often done in secret, gives him a very large circle of friends. Opposing him can be an exceedingly lonely occupation.