Washington Democrats and Libertarians are asking the U.S. Supreme Court to hear yet another challenge of the state’s popular Top 2 Primary.

Various appeals have been under way since voters approved the system by a landslide eight years ago.

The open primary, which allows all voters to select their favorite candidates for each office, without regard to party label, has been successfully used since 2008, when the U.S. Supreme Court ruled 7-2 to allow its use. The high court did leave open the possibility of further challenge based on the way the state administered the winnowing election.

Barring some unexpected development, the state plans to use the Top 2 Primary on Aug. 7 to winnow the field for governor and other statewide offices, Congress, the Legislature and other offices. The two top vote-getters will advance to the General Election in November. No party is guaranteed a runoff spot.

The U.S. District Court and the 9th Circuit Court of Appeals have rejected arguments by the Democratic, Republican and Libertarian parties of Washington that their constitutional rights are violated. The Republicans have dropped out of the challenge. Just before the deadline late Wednesday, the Democrats and Libertarians filed separate requests that the high court hear a further appeal.

Secretary of State Sam Reed, the state’s elections chief, said he was disappointed that the Libertarians and Democrats persist in their challenge, and pleased that Republicans have heeded the request he has made for all three parties to stop challenging a voter-approved system that is working well and producing good candidates and officeholders.

“Our system, which is a model for other states, really honors the way Washingtonians want to vote – for the person, not the party label,” Reed said. “It really fits our populist, independent streak and allows people to split their ticket, rather than be confined to one party’s candidates. The parties’ challenge of our old blanket-primary led to our Top 2 system, with a very unpopular detour to the Pick-a-Party system that limited our primary choices.

“I hope the Supreme Court will decline to take the case, and will acknowledge that we followed the court’s roadmap for how to conduct the primary as a nonpartisan, winnow election that puts the voter in the driver’s seat.”

Attorney General Rob McKenna said the Top 2 system has withstood scrutiny by the voters and by the federal courts, and has passed with flying colors. The state’s implementation followed the Supreme Court’s own guidance for how to design election materials and a filing system that clearly inform the voter how the nonpartisan winnowing process works, he said.