Point out what unions have done historically. Describe how things were in American workplaces before unions. Workers were fired at the whim of management or arbitrarily at the age of 40. Describe the extremely low wages, long hours, no fringe benefits, no unemployment compensation, no Social Security and no workers' compensation. Stress not only the contract gains but also the efforts of labor to enact better laws and create better communities.
Unions are just associations of people banded together for mutual protection and benefit.
The newspapers do their best to make unions look bad, and this is understandable since employers spend a lot of money on ads. I get along fine with my supervisor. But this is no reason why we shouldn't try to make it an even better place to work. Your job is made more pleasant and secure because of the union representation afforded you on the job. Your supervisor has to treat you fairly since the contract requires that he or she do so.