http://www.unionfacts.com/article/political-money/
Every day, millions of union members have money taken from their paychecks to support some union presidents political agenda. In 1996, Rutgers economics professor Leo Troy estimated that union political expenditures totaled about $500 million in each election cycle. More recently, the National Institute for Labor Relations Research estimated that total union political expenditures reached $925 million in the 2004 cycle. Over time, this has added up: According to The Center for Responsive Politics, eight of the top ten all-time political contributors are labor unions.