> Reactance is an unpleasant motivational reaction to offers, persons, rules, or regulations that threaten or eliminate specific behavioral freedoms. Reactance occurs when an individual feels that an agent is attempting to limit one's choice of response and/or range of alternatives.
When I think about the people who I know who are anti-union, they all have high reactance as a trait. If I were to try to change their mind, it would be to frame their non-union environment as more freedom-limiting than the unions alternative re: workplace democracy etc.