Most rank and file union workers (at least the manufacturing type guys rather than the government (non-) worker unions) that I know can be pretty conservative. They supported Regan and they helped elect W twice.
My bet is that the union leadership will not force the issue. Obama has not done manufacturing workers any favors. The NYC Public Safety Unions will have a conniption if this thing is built there.
My fondest hope is that the construction union workers refuse to build it and that Holder starts a suit against them for religious discrimination.
I would LOVE to see the imam and Holder go after them. Blood in the streets.