The Supreme Court ruled today the government may not ban political spending by corporations in candidate elections.

Justice John Paul Stevens read a long dissent from the bench. He said the majority had committed a grave error in treating corporate speech the same as that of human beings. His decision was joined by the other three members of the court’s liberal wing.

The case had unlikely origins. It involved a documentary called “Hillary: The Movie,” a 90-minute stew of caustic political commentary and advocacy journalism. It was produced by Citizens United, a conservative nonprofit corporation, and was released during the Democratic presidential primaries in 2008.