Answer:
The supreme court believed that it violates the First Amendment right of association. "Proposition 198 forces political parties to associate with—to have their nominees, and hence their positions, determined by—those who, at best, have refused to affiliate with the party, and, at worst, have expressly affiliated with a rival," wrote Justice Antonin Scalia. "A single election in which the party nominee is selected by nonparty members could be enough to destroy the party."
