ASHINGTON - Supreme Court justices voiced concerns yesterday that a new California primary law that allows voters to cast ballots across party lines might trample political parties' rights to choose their nominees.
The robust session focused on ''blanket'' primaries, the system in California and three other states that lets voters choose among all parties for each office on an election ballot. For example, voters can choose a Democratic nominee for president, a Republican nominee for governor, and a Libertarian one for lieutenant governor.
An eventual ruling also could affect about 20 states that hold so-called open primaries. Under that system, voters can decide on election day which primary they will vote in.
In last February's Michigan GOP primary, Arizona Senator John McCain defeated Texas Governor George W. Bush with the support of Democrats and independents. McCain, who supports California's blanket primary law, is among the many politicians and political groups who have weighed in with ''friend of the court'' briefs in the closely watched case.
''The very essence of the party's First Amendment right,'' Justice Sandra Day O'Connor said yesterday, ''is to define its own message and decide its own candidates.''
Under the California scheme, asserted Justice Antonin Scalia, voters have ''absolutely no commitment to a political party, not even for a day.'' When California special assistant attorney general Thomas Gede said voters want their candidates to be ''more representative'' of a broader spectrum of the electorate Scalia declared: ''That's democracy carried to the extreme.''
The case pits a state's interest in boosting turnout and offering voters more choices against political parties' ability to choose their nominees. Californians approved the blanket primary in a 1996 ballot measure with nearly 60 percent of the vote, and it took effect in 1998. Only three other states - Alaska, Louisiana, and Washington - hold such primaries.
The state's Democratic Party, Republican Party, Libertarian Party, and Peace and Freedom Party challenged the system as an infringement of their First Amendment freedom to associate. They claimed crossover voting permits outsiders to elect a party's nominee and determine its ideology, saying it threatens party members' ability to advance their common political beliefs.
Lower courts rejected the challenge, concluding that the burden on the parties' rights was minimal and agreeing with the state that the system enhanced the democratic nature of elections.
Their attorney, George Waters, told the justices Monday that ''a party has a right to limit its nominating process to those who affiliate with the party.'' Waters emphasized that people join political parties to express certain ideas and ideologies.
O'Connor, who was Arizona's Senate majority leader in the 1970s and is the only member on the current bench who held elective office, was one of the most vigorous questioners. She specifically asked whether the court would have to invalidate open primaries if it struck down California's blanket system.
Waters said no. He contended that the First Amendment requires party voting to be limited to those who ''affiliate'' with a particular party, and that in an open primary, there is some affiliation, even though it might be limited to the day ballots are cast.
Justice David H. Souter questioned whether a voter who walks into the polls and makes a ''last-minute'' choice of a party slate was truly affiliating with a party. Waters responded that the political parties assumed such voters were choosing their preferred candidates in ''good faith'' rather than trying to undermine the process.
In other action yesterday, the court let South Carolina go ahead with its plan to shut down the $2.8 billion-a-year video gambling industry in that state beginning July 1.
The justices made no comment as they turned down a video gambling company's challenge of the way the ban was approved last year.
