Final answer:
The U.S. Supreme Court ruled blanket primaries unconstitutional in 2000 because they violated political parties' First Amendment right to associate freely and to control their own nomination process.
Step-by-step explanation:
The blanket primary was ruled unconstitutional in the California Democratic Party v. Jones case of 2000 by the U.S. Supreme Court. In a blanket primary, voters can select candidates from any party for each office, regardless of their own party affiliation. The Court's decision rested on the principle that political parties have a First Amendment right to associate freely and to determine the selection of their own nominees. The assertion was that blanket primaries infringed upon the rights of political parties by allowing non-party members to influence candidate selection, effectively diluting the parties' control over their own nomination process. This ruling means states can now only use closed or open primaries as methods for party nomination, both of which limit the influence of non-party members on the nomination of party candidates.