The idea that police and firefighter unions supported him politically is nonsense.
Police & fire may have supported him, for their own reasons: here in NJ, both police & fire personnel ARE being laid off, while the teachers not only hang on but are still getting pay increases (stronger union). Paterson NJ just became another city to lay off 1/3 of its police force (following Camden, Newark, Jersey City); teachers are doing just fine in the meantime. Those unuons stuck together when the money was rolling in; now it is obvious (especially to the police) that they are at the bottom of the food chain.
Fire departments are slightly better off, as there are less paid members; in my town they agreed to concessions/ demotions to avoid layoffs. Our recenly-hired veterans, who were sheduled to be laid off as the most recent hires, attended town hall meetings in their military uniforms to protest the cuts; I guess we owe them a lucrative job for life (in a town with few fires - the large paid department is a holdover from a time when there was a lot of industry which left decades ago).