In the case of EU, SECRETARY OF STATE OF CALIFORNIA, et al. v. SAN FRANCISCO COUNTY DEMOCRATIC CENTRAL COMMITTEE et al., 1988, the U.S Supreme Court ruled on a dispute regarding California's open primary system and its constitutionality under the First Amendment rights to freedom of association. The Democratic Central Committee argued that their right to choose their own party representatives was being infringed upon by allowing non-party members to vote in primaries. However, Secretary of State March Fong Eu defended this practice as it encouraged political participation and prevented corruption within parties. In a unanimous decision led by Justice Thurgood Marshall, the court sided with Eu stating that while political parties do have certain freedoms under the First Amendment these did not extend to controlling who could vote in a "blanket" primary election.

In the dissenting opinion for EU v. San Francisco County Democratic Central Committee, Justice Brennan argued that California's ban on primary endorsements by political parties violated the First Amendment rights of free speech and association. He contended that a state cannot dictate how a party should structure itself or conduct its affairs as it infringes upon their freedom to associate for the advancement of common political beliefs, which is at the core of our electoral process and central to our system of government. Furthermore, he disagreed with majority’s view that banning endorsements protected against corruption and maintained party neutrality in primaries; instead asserting there was no compelling evidence supporting these claims. Therefore, according to him, such restrictions were not narrowly tailored enough to justify infringement on constitutional rights.