The U.S. Supreme Court on Nov. 8, 2016, in Washington. (Zach Gibson/Getty Images)

This week, federal election commissioner and former commission chair Ann Ravel publicly announced her upcoming resignation. She didn’t mince words: “The mission of the FEC is essential to ensure a fair electoral process. Yet since the Supreme Court’s Citizens United decision, our political campaigns have been awash in unlimited, often dark money.”

Citizens United is one of the most controversial Supreme Court rulings of recent years. Issued in 2010, it establishes that “outside spending” in elections qualifies as constitutionally protected speech, effectively removing restrictions that date back to 1947. As a result, corporations and unions have the right to spend unlimited (and largely undisclosed) amounts of money advocating in favor of or against specific candidates. Many, including President Barack Obama, have disagreed with the decision. During the past presidential campaign, Donald Trump repeatedly endorsed this view, referring to the super PACs which emerged as a result of Citizens United as a “total phony deal.” Calls for change have also come from others within the Republican Party.

Could President Trump lead an effort to reform campaign finance? There’s one challenge: Our recent research shows that Citizens United has earned Republicans a substantial number of state legislative seats.

Our research focuses on state legislative elections because we can more easily isolate the effect of Citizens United compared with other factors that influence election outcomes at various levels (such as the popularity of the president). Before 2010, 23 states had bans on corporations and union funding of outside spending. As a result of the court’s ruling, these states had to change their campaign laws. We can then compare the changes before and after Citizens United in these 23 states with the same changes in the 27 states whose laws did not change. The effect of the court’s ruling is then simply the differences between these two before-and-after comparisons.

We find that Citizens United increased the GOP’s average seat share in the state legislature by five percentage points. That is a large effect — large enough that, were it applied to the past twelve Congresses, partisan control of the House would have switched eight times. In line with a previous study, we also find that the vote share of Republican candidates increased three to four points, on average.

We also uncovered evidence that these results stem from the influence of corporations and unions. In states where union membership is relatively high and corporations relatively weak, Citizens United did not have a discernible effect on the partisan balance of the state legislature. But in states with weak unions and strong corporations, the decision appeared to increase Republican seat share by as much as 12 points.

Citizens United also changed state legislatures in other ways. First, state legislatures became more conservative after the ruling, and more so in states with relatively weak unions and strong corporations. Second, the ruling appeared to produce a small increase in the ideological extremism of representatives. Surprisingly, this effect is stronger for elected Democrats, who tend to become more liberal, than for elected Republicans.

Despite the outcry that followed the ruling, some observers were quick to point out that wealthy interests already had many avenues of influence in elections. So it is too early to claim that Citizens United completely reshaped corporate influence in U.S. politics.

Nevertheless, our findings show that allowing corporations and unions to directly spend in elections has had important consequences. This makes it especially interesting to see if Trump follows up on his criticisms of the campaign finance system. Ravel’s replacement might give an early signal of his intentions.

Nour Abdul-Razzak is a PhD student at the Harris School of Public Policy, University of Chicago.

Carlo Prato is an assistant professor in the Department of Political Science, Columbia University.

Stéphane Wolton is an assistant professor in the Department of Government, London School of Economics and Political Science.