The Democratic Party of Hawaii argues in a new legal filing that there is no important difference between Hawaii’s open primary and a California blanket primary that the U.S. Supreme Court ruled was unconstitutional in 2000.

The party has told the 9th U.S. Circuit Court of Appeals that the open primary — like a blanket primary — gives Democrats no alternative for nominating candidates, allows all registered voters to participate regardless of party affiliation, and does not let Democrats learn the identities of the voters who have chosen to select their nominees.

The only difference, according to the party, is that the open primary requires voters to secretly choose candidates from a single political party in all races on the primary ballot, while in a blanket primary, voters can pick candidates from multiple political parties down the ballot.

"It is upon that single slim thread that the whole purported distinction between ‘blanket’ and open primaries depends," the party wrote in the legal filing.

The Democratic Party of Hawaii has challenged the state’s open primary as a violation of the First Amendment right to free association. The party wants to restrict primaries to party members and voters who publicly choose to affiliate with the party before the elections.

In November, U.S. District Judge J. Michael Seabright ruled that Democrats failed to prove that the open primary places a "severe burden" on the free association rights of political parties, because some parties might want all voters to be able to participate.

The judge also found that the party failed to present evidence that open primaries have invited crossover or independent voters to select candidates that have influenced the party’s message.

The party has appealed Seabright’s ruling to the 9th Circuit in San Francisco. Democrats outlined their legal argument in a filing in late March. The state Attorney General’s Office, which is representing Scott Nago, the state’s chief election officer, has received an extension until late May to file the state’s argument.

"We are looking for A. — if possible — a victory, B., if not a victory, then at least clarification of the ground on which the next fight has to take place," said Tony Gill, an attorney for the Democratic Party of Hawaii.

The difference between an open primary and a blanket primary could be critical.

The U.S. Supreme Court ruled in California Democratic Party v. Jones in 2000 that California’s blanket primary was an unconstitutional violation of the First Amendment right to free association. The court held that the blanket primary took away a political party’s basic function of choosing its candidates for general elections.

Voters in Hawaii adopted an open primary at the recommendation of the 1978 Constitutional Convention. The state maintains that the open primary protects voter privacy and removes barriers to voting.

But Democrats have argued in their legal filing that they wish to reserve primaries "to those who share the interests and persuasions that underlie DPH’s being.

"Beyond any doubt, Hawaii’s open primary law intentionally prohibits DPH from doing this, rendering freedom of association an empty guarantee."

Democrats allege that the open primary may actually encourage more damaging crossover voting than a blanket primary, because voters, perhaps lured by one or two interesting races, are then stuck with the party’s candidates for the remainder of the ballot.

The party also argues that its legal challenge, which contends that the open primary is unconstitutional in all applications, should not fail simply because other parties might not object. Few legal challenges would ever succeed, the party argues, if the legal standard is that others might consent to the laws.

Democrats have also opted not to present evidence about voter behavior in open primaries, citing that the state has not maintained data on voter party affiliation since the conversion to an open primary three decades ago. Along with the expense of compiling such voter data from scratch, the party argues that its free association rights should not depend "on the fluctuating and irrelevant behavior of others."

Dante Carpenter, the Democratic Party of Hawaii’s chairman, said he had tried to get the Hawaii Republican Party and the Libertarian Party of Hawaii to support the legal challenge, but that the other parties were not interested.

Carpenter acknowledged that the legal principle the party is pursuing might be "hard to convey to the guy on the street."

Several Democrats contend the party is damaging its public image and risks alienating voters who have overwhelmingly kept the party in power since statehood.

Many of the party’s elected officials have opposed the legal challenge as contrary to the party’s message of inclusiveness.

"I don’t think it’s a good idea," said state House Majority Leader Scott Saiki (D, Downtown-Kakaako-McCully). "We don’t want to exclude people."