A union could raise wages without causing unemployment of union members if it can increase demand for union labor. How might this goal be achieved?

If unions can increase the labor productivity of the workers, then the demand for labor would increase. Perhaps the union workers are more productive because they don’t have to worry about being fired for no cause since the union is looking out for their interests. Perhaps the unions can induce consumers to demand union-made goods through advertising campaigns, or can decrease demand for nonunion made goods by getting tariff legislation passed.