[WAthrowaway]

The more people that could be even potential union members, the stronger the union's negotiating position is.

[manfredo]

This is not always the case. If there are a large supply of workers then many will be willing to work for non-union employers.

In many cases, unions increase the value of labor by constricting the supply of labor. Unions can often be difficult to join. If there is a overabundance of labor, then the union has little negotiating power.