Many things determine who wins on Election Day, and simple correlations don’t automatically indicate causal effects. States that were forced to lift their bans on independent expenditures may have elected more Republicans in 2010, but this surge could have been caused by a reaction to the passage of the Affordable Care Act, the recession or any other issue on voters’ minds at the time. And while states with independent expenditure bans may have elected more Democrats before 2010 than did states without such bans, this does not necessarily have anything to do with how the states regulated election finance. There are many reasons some states vote differently from others, including historical, cultural and demographic differences….

With this approach, we found that the chance of Republican candidates winning state legislative seats increased by about four percentage points on average as a result of Citizens United, and by 10 or more percentage points in several states. The decision also made it more difficult to unseat Republican officeholders, cementing the already strong financial advantage of political incumbents, and reduced the number of Democratic candidates who ran for office. Finally, the data provide evidence that Citizens United discouraged ordinary people from making monetary contributions to candidates’ campaigns, an effect feared by critics of the decision early on.