Michael McConnell of Stanford Law School has a very informative Wall Street Journal op-ed on the impact the Citizens United decision has had on U.S. politics:
Speaking generally, Citizens United is likely to benefit Democrats more than Republicans. Corporations rarely make independent expenditures during candidate elections in their own name, because the ads offend customers, workers and shareholders. And direct corporate contributions to candidates tend to be split more or less evenly between the two parties, largely neutralizing their effect.
But unions have no compunctions against running campaign ads, and almost all of their money goes to Democrats. The Republicans’ advantage, when they have one, comes from rich individual donors—and the right of individuals to make expenditures in support of candidates long predates Citizens United.
McConnell’s view isn’t universally hold, but it seems entirely consistent with the facts.