A report by the Wall Street Journal looks at the amount of money spent by unions on political organizing and finds it is actually four times the amount reported in FEC filings. Since 2005, unions have spent $4.4 billion on direct donations and organizing activities, nearly all of it on behalf of Democrats.
The Federal Election Commission requires unions to report any direct donations to candidates or PACs. However, more detailed reports filed with the Department of Labor indicate that the money reported to the FEC is only about 1/4 of the amount unions spend on political activity. Money spent on things like internal communications to members (pushing them to vote for union approved candidates) and local political races represent the remainder.
About half of the money reported to the Labor Department is spent on donations to state and local races. The remainder is spent paying salaries of union officials who spend their time on political activities, nearly all of which benefits Democrats. In 2010, union political work contributed the equivalent of "3,242 full-time operatives with a payroll of $214 million."
The donation of time and money to Democratic candidates by unions makes up for the contributions of corporations. As the Journal notes, corporate donors tend to split their money between the parties, "In 2008, Democrats received 55% of the $2 billion contributed by corporate PACs and company employees, according to the Center for Responsive Politics." The net effect, when it does benefit Republicans, is smaller than the donation of union time and money, more than 90 percent of which goes to Democrats.
The AFL-CIO is one of the largest union donors. Combining the money spent on the losing (for Democrats) 2010 election cycle and the losing 2011 recall election in Wisconsin, the AFL-CIO has dropped nearly a billion dollars in the last three years. Though the AFL-CIO has lost 2 million members since 2005, the percentage of dues spent on politics has gone up during this same period.