In its recent decision in California Democratic Party v. Jones, the Supreme Court struck down California’s “blanket primary,” which allowed any voter to vote in any race in any party’s primary. The decision has propelled questions of primary voter qualifications to the forefront of constitutional analysis of political parties. This Article analyzes the case law on state regulation of primary elections and argues in favor of constitutional protection for party organizational autonomy in determining qualifications for primary voters. Legal scholars have been almost unanimous in their condemnation of the Court’s decision in Jones. This Article takes a different view. Agreeing with the critics that traditional First Amendment rights of expression and association largely are inapplicable to party primaries, this Article advocates an approach that pays less attention to parties’ status as state actors or private associations and more attention to the functions they play in American democracy. In particular, the Article argues that autonomous parties are a necessary check against one party’s manipulation of the electoral process to its advantage and an indispensable means of aggregating interest groups into the American political system. Recognizing that in the context of primary elections, today’s major political parties are, nevertheless, state actors, the Article concedes that explicit constitutional guarantees preventing discrimination in the right to vote ought to apply to major-party primaries.