The Center for Public Integrity has crunched the numbers:
According to the records of the Federal Election Commission, $933 million came directly from the companies, unions and individuals who took advantage of Citizens United to funnel money into super PACs. This money overwhelmingly went to paying for the attack ads that made the 2012 election one of the most historically bitter contests, with pundits regularly calling it the “nastiest,” “meanest,” or even “dirtiest campaign in history.”
The FEC records show that about two-thirds of all the Citizens United-fueled money went to ten super PACs or political nonprofits, nine of which focused exclusively on buying media spots and ads for candidates. Of these ad spots, 89 percent were focused on denouncing the opposing candidate, explaining the prevalence of attack ads throughout the contest.
Political consultants have long argued that attack ads are more powerful in swaying voter opinion–and the recent outpouring of money into attack ads offers further evidence of negativity’s success. Many consultants also think, however, that these same attack ads adversely affect the democratic process. In one study, about 75 percent of consultants polled “contend[ed] that attack ads and negative TV advertisements contribute to voter cynicism throughout the campaign season.”
You can read the CPI’s summary report here.