www.publicintegrity.org/2014/01/16/14107/top-us-corporations-funneled-173-million-political-nonprofits
"Many such trade associations and so-called “social welfare” groups have released a tsunami of political ads since the Citizens United decision. These funds have been dubbed “dark money” because nonprofits organized under sections 501(c)(4) and 501(c)(6) of the U.S. tax code need not publicly disclose the sources of their funding — unlike candidates, political parties and super PACs."
