Pamela S. Karlan in the Boston Review:
When the Supreme Court heard Santa Clara County v. Southern Pacific Railroad Co. in 1886, few would have pegged the case as a turning point in constitutional law. The matter at hand seemed highly technical: could California increase the property tax owed by a railroad if the railroad built fences on its property? As it turned out, the Court ruled unanimously in the railroad’s favor. And in so doing, the Court casually affirmed the railroad’s argument that corporations are “persons” within the meaning of the Fourteenth Amendment, which provides that no state shall “deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.” So certain were the justices of the Fourteenth Amendment’s applicability that their opinion did not engage the issue, but the Court reporter recorded the justices’ perspective on the topic:
Before argument Mr. Chief Justice Waite said: ‘The Court does not wish to hear argument on the question whether the provision in the Fourteenth Amendment to the Constitution which forbids a state to deny to any person within its jurisdiction the equal protection of the laws applies to these corporations. We are all of opinion that it does.’
That statement marks the origin of the view that corporations are persons as a matter of constitutional law. This played a central role in the 2010 decision in Citizens United v. Federal Election Commission, which struck down portions of the Bipartisan Campaign Reform Act that restricted corporate spending on electioneering communications in the run-up to a federal election. The Court declared that Congress could not discriminate between electioneering communications according to the identity of the speaker: since individual human beings clearly have a First Amendment right to speak about candidates during the election process, so too must corporations.