Since 1976, when the Supreme Court decided the seminal case of Buckley v. Valeo, the Justices have been locked in what both sides see as a Manichean struggle over the constitutionality of campaign finance regulation. On one side are those Justices who view the world of politics as fraught with corruption and undue access for the wealthy; they worry that voter confidence gets shaken by each new campaign finance scandal. On the other side are those Justices who see any limitation on money in politics as overt government censorship that violates the First Amendment; they fear that incumbents will squelch criticism in a replay of the Alien and Sedition Acts. Justices fight this rarified battle with jurisprudential jargon that parses levels of scrutiny, compelling interests and the appropriate tailoring of the law, but it is this fundamental difference in worldviews that really drives the Court’s debates. And as Court personnel shifts—or, less often, Justices change their minds—the Court’s doctrine swings like a pendulum, alternating between deference and skepticism toward the regulation of campaign finance.

To most Americans, the Supreme Court’s 5–4 decision in Citizens United v. Federal Election Commission this past January, which recognized a constitutional right of corporations and labor unions to spend unlimited sums in candidate elections, came as a bolt from the blue. But for those who follow the issue closely, the decision was an inevitable consequence of the retirement of perennial swing-voter Justice Sandra Day O’Connor and her replacement with the more conservative Justice Samuel Alito. The outcome of Citizens United, the Court’s most skeptical judgment ever on the constitutionality of campaign finance regulation, came just a few years after the Court’s most deferential decisions in the area.

How did we get here? What did the Court actually do in Citizens United? And what room does the Citizens United decision leave for future regulation?