If you said sneaky right wing nuts, the "1 percent" (i.e. superrich Americans) or corporate powerhouses, you'd be wrong. The biggest hunk of money came from Planned Parenthood Action Fund Inc., yeah them, the abortion industry. The second biggest chunk came from our friends at American Federation of State, County and Municipal Employees. Interestingly, the bulk of the money those two groups spent was for opposing Republicans (otherwise known as negative advertising) and relatively little on supporting their Democratic candidates. And how did that spending go for you in Wisconsin, AFSCME?
You can see the rest of the study by the Sunlight Foundation of the undisclosed funding here.