Not sure that a labour union is actually considered a special interest group within the broader political culture- especially the NDP which is essentially a Labour party in the truest sense of the word. The CAW union is more like an organic element to the NDP, but one must take care to separate union interests from union political interests and unions (generally) from union members (generally). Most male, blue collar union members tend vote Conservative, when they bother to vote at all.
For example, I am 99% positive the CAW workforce at GDSL in London or the Ford plant in Talbotville are not in favour of gay marriage, the Kyoto protocol, higher taxes for social programs, open immigration and a number of other issues important to the NDP and the CAW.
What they do want is the right to keep most of their paycheck, the ability to be treated in the hospital when they are sick, see that their kids get a decent and realistic education that focuses on the necessary skills to do well in life, they want to see criminals go to jail under sentences that fit the crime, they want to work a decent work week for a decent living without fearing they will lose their job to a scab or low cost worker in another country - any country. In these respects, and many others, unions have failed their workers because unions back the wrong political horse.