The following chart from the Center for Responsive Politics represents outside spending by non-candidate and party groups seeking to influence elections in every election cycle going back to 1990. As it demonstrates, such spending was on the rise even before the Supreme Court opened the floodgates to unlimited corporate influence on elections in the Citizens United decision. Nevertheless, another trend is also clear. Prior to Citizens United, which was decided in 2010, left-leaning groups held a moderate-to-significant advantage in election spending. After Citizens United, conservatives absolutely dominated the field:
To be fair, some of the massive disparity in 2012 can be attributed to the contested Republican primary — and was spent on Republican-on-Republican hits rather than on attempts to improve Republicans’ chances against Democrats. Nevertheless, the last two election cycles suggest that conservatives will continue to benefit from Citizens United even once the general election kicks into full gear. Citizens United gave such a boost to Republican candidates that outside spending by conservatives grew by more than $70 million from 2008 to 2010, even though 2008 was a presidential election year and outside spending has historically been much higher in these cycles than in off-year elections.