The system allows candidates to indicate their party affiliation on the ballot and is open to voters regardless of their party. The system is the successor to the blanket primary that the Supreme Court struck down in 2000 on the grounds that it violated political parties' rights.
The Washington Democratic and Republican parties sued to block the new system, approved by voters in 2004. The parties said they would be forced to be associated with candidates they did not choose.
Lower federal courts agreed with the parties. The new system, which has never been used, infringes on the rights of political parties to choose their own nominees, the 9th U.S. Circuit Court of Appeals said.
"The net effect is the parties do not choose who associates with them and runs using their name; that choice is left to the candidates and forced upon the parties," Judge Raymond Fisher wrote.