How has the U.S. Supreme Court ruled on political parties’ right to control their candidate nomination process?

In California Democratic Party v. Jones (530 U.S. 567, 2000), the Supreme Court ruled 7–2 that political parties have a First Amendment right of association to control their own candidate-selection process. The Court struck down California’s “blanket primary” law, which allowed non-party members to vote in party primaries, because it forced parties to associate with candidates and voters they did not choose. The decision affirmed that parties may exclude or vet candidates to protect their message and identity — a principle that supports local committees’ authority to set and enforce vetting requirements.

The ruling affirms that parties may set their own rules — including vetting — to control who uses the party label on the ballot, but it does not give parties the power to override voter choice in the primary election itself.