> A union is a mechanism by which a class of people with less power (typically workers) pool together to enforce rules on people with more power (typically employers).

Only if you ignore the fact union members are giving up power to union leaders, who can force them to give money to support political candidates as a condition of employment and who can, as part of a contract, deny them the ability to participate in labor actions if the union leadership disagrees, probably for some profit motive.