Open Primary Trial in Idaho
The three-day federal trial in the Idaho Republican Party's lawsuit against the state-mandated open primary concluded on October 15. The Republican Party wants to be able to enforce its rule that says that only Republicans may vote in GOP primaries. According to Ballot Access News:
"A last round of briefs will be filed in the next few weeks. The decision, when it comes down, will be a landmark. This is the first case in which either major party has submitted empirical evidence that it needs to close its primary because persons not in sympathy with the party have been voting in its primaries."
The Republican Party presented evidence that 41 percent of the voters in the 2008 GOP primary were non-Republicans.
"The Mississippi Democratic Party case [against the state-mandated open primary] had lost on a legal technicality..."
The technicality was that the Democrats had not adopted a party rule for a closed primary. The U. S. district court declared the open primary law unconstitutional, but in May 2008, the 5th U. S. Circuit Court of Appeals in New Orleans reversed the district court (Mississippi Democratic Party v. Barbour, 07-60667).
Dennis Mansfield attended the Idaho trial and produced several blog posts on it, starting on October 13. Another interesting post is here (Dennis's other posts on the trial are commingled with posts on different topics).
If the state-mandated open primary is ultimately struck down, each party will have the option of inviting independents to vote in its primaries (Tashjian v. Republican Party of Connecticut, 1986).
Unless the state prohibits it, each party will also have the option of inviting members of opposing parties to vote in its primaries-- which the Idaho Republicans obviously won't do (Clingman v. Beaver, 2005).
 Each voter chooses a party on primary day.