What makes super PACs super?
Getting rid of the $5,000 limit was essential, as the great majority of super PAC funds come from individuals. The Citizens United grant of corporate First Amendment rights to make independent expenditures was not enough by itself to create super PACs. For-profit business corporations neither made their own independent expenditures nor, as far as we know, contributed much to PACs that did. But 501(c)(4) social welfare groups, which are nonprofit corporations, did both.
Super PACs are just super-sized versions of the kinds of political committees that had been regulated under the FECA since 1971, so they still had to report their donors to the FEC. That is not true of 501(c)(4)s; they can hide their donors, which made them ideal companions for super PACs.
Republican political strategist Karl Rove was one of the first to see the advantage of pairing a super PAC with a 501(c)(4). His American Crossroads was the first big super PAC, formed just three weeks after SpeechNow. He had spent months pitching the idea for American Crossroads to the GOP's biggest donors, and they had pledged to contribute $30 million. But in the first month after launching the PAC it had received only $200. Donors still wanted to honor their pledges, but they were reluctant to go public. Rove solved that problem by creating a sister organization, a 501(c)(4) called Crossroads Grassroots Policy Strategies. That did the trick. The American Crossroads/Crossroads GPS combination raised $325 million in 2010, which at the time was a record for a non-party group.5
It was not long before someone tried to super-size a traditional PAC by creating a hybrid PAC.