By Ilya Shapiro/Cato Institute
As recent events in Wisconsin have demonstrated, public-sector unions are powerful political constituencies that can shape government to their ends. The Service Employees International Union, for example, the defendant in Knox v. SEIU Local 1000, has been ranked by OpenSecrets.org as the fifth biggest “heavy hitter” in federal politics in terms of campaign spending.
In 2005, the SEIU initiated a mid-year campaign against two California ballot measures, one that would cap state spending and another that would restrict the use of union dues for political purposes. In states such as California that do not have “right to work” laws, unions are allowed to take dues from non-union workers to finance collective-bargaining activities that, arguably, benefit all employees. Since 1977, however, unions have not been allowed to take dues from non-union members to pay for pure political advocacy without adequate protections for possible dissenters.
http://www.cato-at-liberty.org/unions-cant-force-non-members-to-pay-for-political-advocacy/