Much has been written about contributions from corporations to candidates for public office. That coverage tends to focus more heavily on the spending of corporations.

According to data tracked and publicized by the Center for Responsive Politics, however, it would appear that union contributions to political candidates are just as heavy as corporate contributions, at least among the biggest spenders. Four of the top 10, and half of the top 20, political action committees (PAC) are union-related. The Electrical Workers lead the pack, having donated $2,561,123 to political candidates from 2009 to 2010.

The total amount contributed to candidates for public offices by union PACs during this time period was $18,264,058. In some cases, for example, the American Federation of Teachers and the American Federation of State, County and Municipal Employees (AFSCME), 100% of those donations went to the Democratic Party's candidate.

Non-union PAC contributions in this top 20 category also tended to favor the Democratic Party's candidate, although not by such a substantial margin. Of the top 20 PACs, only nine could be said to represent "corporate" interests. One, the American Association for Justice, is the PAC for trial lawyers.