Over the weekend, Matt Bai claimed that Citizens United has little to do with it:
The level of outside money increased 164 percent from 2004 to 2008. Then it rose 135 percent from 2008 to 2012. In other words, while the sheer amount of dollars seems considerably more ominous after Citizens United, the percentage of change from one presidential election to the next has remained pretty consistent since the passage of McCain-Feingold. And this suggests that the rising amount of outside money was probably bound to reach ever more staggering levels with or without Citizens United. The unintended consequence of McCain-Feingold was to begin a gradual migration of political might from inside the party structure to outside it.
Rick Hasen counters, noting that Citizens United eliminated the risk of criminal investigation for large donors:
If Sheldon Adelson really was planning on giving $100 million to 527s before the Citizens United revolution to support a presidential candidate, you can bet that there would be a criminal investigation and very serious charges considered. [I] have serious doubts Adelson or anyone else would have risked this (much less corporations giving considerable sums to 501c4s for election-related activity).
Mark Schmitt adds important context:
Bai’s stale claim that Citizens United isn’t largely responsible for the explosion of outside money in politics is no longer credible. I realized that in November of 2010, when I wrote a piece with the title "The Re-Education of a Citizens United Denier." At the most basic legal level, Citizens United formed the basis for the D.C. Circuit Court’s decision in SpeechNow vs. FEC, and thus there is a direct path from CU to Super PACs…. Focusing only on the presidential years, he points out that outside spending rose by a greater percentage from 2004 to 2008 (both years before CU) than between 2008 and projected 2012…. Between 2004 and 2008, there was also a significant legal change, the Wisconsin Right to Life decision, which half-opened the door that Citizens United tore down.
(Chart from Open Secrets)