The Supreme Court agreed Tuesday to decide if political parties can open their primaries to all voters, in a constitutional challenge to Oklahoma's voting system.

Oklahoma has a semi-closed primary arrangement, in which political parties may allow only their own members and voters registered as independents to cast ballots.

The Libertarian Party sued, claiming the restrictions infringed on its First Amendment (search) free association rights. The party wanted to open its primaries to Oklahoma voters registered as Democrats and Republicans, in hopes of attracting more members.

Oklahoma argued that opening the system would undermine its election process, but the 10th U.S. Circuit Court of Appeals disagreed.

Wellon Poe, a state assistant attorney general, told the justices in a filing that the appeals court decision would require states to allow open primaries, in which people can cast votes in any primary. He said that infringes on a state's authority to regulate elections.

The justices will hear Oklahoma's appeal early next year.

The Supreme Court last dealt with the issue of primaries four years ago and ruled that political parties could not be forced "to associate with those who do not share their beliefs."

Richard Hasen, an election law expert at Loyola Law School, said this case gives the court an opportunity to clarify the rights of parties that want open voting but are barred by state rules.

The case is Clingman v. Beaver, 04-37.