Thanks to a recent Supreme Court decision on public sector union membership, many members of Local 925 of the Service Employees International Union — those who had not explicitly signed up to join the union — suddenly found that dues were no longer being taken directly out of their paycheck. As a result, 38 percent of the local's membership was gone overnight.
Most people do not realize the extent to which unions dominate special interest spending on elections. When you talk about special interests, it is hardly worth mentioning any other than unions. This chart, from Open Secrets, shows the fifteen largest contributors to federal campaigns during the period 1989-2012. Ten of the fifteen are unions.
Unions have chosen to turn themselves into an arm of the Democratic Party rather than represent their members.