This article by Timothy Noah, which appears in The New Republic, is an unusually clear article about campaign finance in federal elections. It illustrates that large for-profit corporations have generally not spent money commenting on candidates for federal office, even though Citizens United v FEC permits them to do so.
The article explains that the large spending for Super-PACS is from individuals, and is a result of the U.S. Court of Appeals, D.C., Circuit decision in Speechnow.org v FEC. That decision was handed down on March 26, 2010, and was a unanimous ruling of all the full-time U.S. Court of Appeals judges in that circuit. Speechnow v FEC lets individuals give as much money as they wish to committees that make independent expenditures about candidates for federal office.
The only groups in the U.S. that can’t receive unlimited donations to make independent expenditures are political parties that have been recognized by the FEC as national committees. There are eight such national committees: Democratic, Republican, Libertarian, Green, Constitution, Socialist, Reform, and Natural Law. Americans Elect is not a “national committee”.
As the Noah article explains, in the past, before the McCain-Feingold law put limits on political parties, rich individuals made large donations to political parties, but that is now illegal, so instead they give it to PAC’s and other non-party organizations. Thanks to Rick Hasen for the link.