In my (always..heh) humble opinion, the California Correctional Officers Assocation is the most powerful union in the USA outside of the Major League Baseball Players Assocation and the NBA players union.
California has the highest per capita incarceration rate in the entire free world
This is from wikipedia so take it for what it's worth:
"The CCPOA is deeply involved in a variety of political activities. Most spending is done through political action committees. Although its membership is relatively small, representing only about one tenth the membership of the California Teachers Association, CCPOA political activity routinely exceeds that of all other labor unions in California. The union spends heavily on influencing political campaigns, and on lobbying legislators and other government officials. CCPOA also hires public relations firms and political polling firms."