Companies with union labor may bid out labor contract to other unions. Members/Laborers become employees of the union who are responsible for their safety, welfare, and pay/benefits as well as being accountable for their performance.
Upon the completion of a union contract, company may negotiate with alternate unions in order to achieve the best contract to mutually benefit both company and union (and its members). Members are free to leave and join the union that best represents them.
Contracts include fines and penalties for either party violating terms of agreement.