A few years back, I was working for a union machine shop, after a few months I got called into the bosses office, I was told he got a call from the union, and that if I didn't pay the $100 initiation fee and $150 in back due rent by the next day, the company would have to fire me.
Illinois and Missouri are NON right to work states. In almost all circumstances where a union is present, it is a closed shop. Meaning that, if a union represents a position or job title all employees performing that tasks must be union. There is an option to pay them slightly less in dues, as a "collective bargaining" fee, but then you are denied the right to vote under the unions bylaws.