The justices' 7-2 ruling threw out California's "blanket primary" system, similar to laws in three other states. The decision avoided deciding the validity of the more common open primary system used in another 20 states.

Allowing nonparty members to help choose a political party's nominees in the manner used by California violates parties' free-association rights under the Constitution's First Amendment, the justices said.

The three states with voting laws similar to California's blanket primary are Alaska, Washington and, to some extent, Louisiana.

The open primary system used in 20 states allows voters to choose which party's primary they will vote in, even if they are not enrolled in that party.

Writing for the court, Justice Antonin Scalia said California was "forcing political parties to associate with those who do not share their beliefs. And it has done this at the crucial juncture at which party members traditionally find their collective voice and select their spokesman."

"The burden (California's voting system) places on (the political parties') rights of political association is both severe and unnecessary," Scalia wrote.

Scalia said states could hold a nonpartisan blanket primary, in which voters can choose any candidate regardless of affiliation, and the top two vote-getters move on to the general election. The system he described would be similar to Louisiana's system.

Under such a voting plan, Scalia said, "primary voters are not choosing a party's nominee" and therefore political parties' rights of free association are not harmed.

His opinion was joined by Chief Justice William H. Rehnquist and Justices Sandra Day O'Connor, Anthony M. Kennedy, David H. Souter, Clarence Thomas and Stephen G. Breyer.

Dissenting were Justices John Paul Stevens and Ruth Bader Ginsburg.

Four California political parties, including the Democrats and Republicans, challenged the blanket primary system overwhelmingly approved by the state's voters in 1996. Until then, Californians could vote only in their own party's primary.

Blanket-primary supporters said the system would encourage voter turnout and lead to the nomination of more moderate candidates. But the four parties said allowing nonparty members to vote would harm their members' ability to choose candidates that best represent their views.

In a blanket primary, everyone receives the same ballot and someone could vote, for example, to nominate a Republican candidate for governor, a Democrat for senator and a Libertarian for state attorney general.

Twenty states have open primaries that let voters choose on election day which party's primary they will vote in. Voters can select among that party's candidates only.

Thirteen states have closed primaries that allow only party members to vote in each party's primary. Another 13 states have closed primaries but also let independent voters cast ballots in at least one party's primary.

The California Democratic and Republican parties were joined by the Libertarian and Peace and Freedom parties in challenging the state's blanket primary. All four said their bylaws bar people who are not enrolled members from voting in their primary elections.

A federal judge ruled against the parties, and the 9th U.S. Circuit Court of Appeals agreed, saying the state had a substantial interest in boosting voter turnout and giving voters a greater choice.

In appealing to the nation's highest court, the parties' lawyers said the California system was an assault on their ability to choose nominees who represent their ideology.

Among the friend-of-the-court advice the justices received in the case was a pro-blanket-primary brief from Sen. John McCain of Arizona, who relied heavily on Democrats and independents in his losing campaign for the Republican presidential nomination this year.

McCain got strong boosts from victories in New Hampshire and Michigan, states with open primaries, but in California he finished third behind Democrat Al Gore and Republican George W. Bush.

The Supreme Court on Monday reversed the 9th Circuit court.

"As for affording voters greater choice, it is obvious that the net effect of this scheme ... is to reduce the scope of choice, by assuring a range of candidates who are all more centrist," Scalia said.

Stevens, writing in dissent for himself and Ginsburg, said the court should "respect the policy choice made by the state's voters" in approving the proposition.

Washington state's blanket primary system dates back to 1935, and Alaska has used a similar system for most years since 1947. In Louisiana's blanket primary system, the top two vote-getters, regardless of party, go on to a runoff election.

The case is California Democratic Party v. Jones, 99-401.

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