Just how much has Citizens United altered the electoral landscape? While the overall amount of outside money in politics has risen dramatically with each passing election cycle, the Citizens United decision eliminated restrictions on corporate and special-interest spending to influence or elections. The result shows that the new rules have stacked the deck in favor of Republicans.
This chart  from the Center for Responsive Politics shows how, while spending by outside groups has been on the rise since the 90’s, it was not until 2010 Supreme Court decision that conservatives saw a sudden, major advantage in outside-expenditure spending on their behalf:
Total Liberal vs. Conservative Outside Spending, Excluding Party Committees
As Ian Milllhiser at Think Progress notes , the much of the 2012 spending on Republican candidates went to intra-party contests during the primaries, though it will likely continue through the general election. But the overall trend is clear: As a result of the Citizens United, Republicans will continue to enjoy outsized spending on their behalf by corporations.