The Washington Examiner's Sean Higgins writes up the effect of Citizens United on Election 2012. The short version: Lots of rich people and unions wasted a lot of money.
Citizens United did allow corporations and deep-pocketed donors to pour lots of money into a campaign … much of which was simply wasted.
Among the presidential campaigns, the party committees and outside groups trying to affect the presidential race, the Republican side raised $1.03 billion this election cycle. The Democrats were very much in the same ballpark at $932 million. In short, Citizens United did not put the Democrats at a steep fundraising disadvantage. In fact, Obama's campaign ended with $93 million still in the bank—he ended up raising more money than he could actually spend.
One of the things often overlooked about the Citizens United decision was that it removed campaign restrictions on Big Labor too. That balanced out a lot of the corporate spending. Unions represented seven of the top 20 donors, both to candidates and tax-exempt 527 fundraising groups.
And watch this way-back-when 3 Reasons Not to Sweat Citizens United: