The citizens of California adopted Proposition 198, which changed the state’s primary system to an open, or blanket, primary, which meant that each voter could vote for any candidate in the primary regardless of party affiliation. The four major political parties in California (plaintiffs) each had a rule prohibiting individuals that were not members of their respective parties from voting in their primaries. The plaintiffs brought suit, alleging that Proposition 198 violated their First Amendment freedom of association rights. The district court ruled against the plaintiffs and the United States Court of Appeals for the Ninth Circuit affirmed. The United States Supreme Court granted certiorari.