Unions tend to be less successful at raising wages in competitive industries and more successful in heavily regulated, monopolistic industries. Unions have little effect on wage in textile industries, but they have a greater effect on wage in autos, steel, mining, and transportation. To be successful in increasing wage while at the same time avoiding excess labor supply, the union must shift the supply curve of labor back. The union must be able to force all employers in the industry to hire only union members. The union can restrict membership by having high initiation fees, long apprenticeship periods, qualification exams, and restrictive licenses.
Unions have a difficulty requiring all firms to hire only union workers.
Restricting supply is a characteristic of craft unions like carpenters, plumbers and brick layers. A way to increase the wage is to increase the demand for union labor. The labor demand curve must shift out. This is attractive because it not only causes wage to increase, but it increases employment. Look for the union label when you are buying a coat, dress or blouse.