The primary mechanism unions use to keep wages high for union members is to limit supply by lobbying to keep "un-certified" or "non-union" members out of the market. This provides a nice buffer, keeping the poorer class poor and protecting the middle class.
Nobel Economist Milton Friedman, for one. https://www.youtube.com/watch?v=xzYgiOC9cj4 If you watch to the end he fully explains how the net effect of unions is to raise the pay of high paid workers and lower the pay of low paid workers.
You might also like "Basic Economics" by Thomas Sowell which talks about this phenomenon also.
