A constitutional challenge to Montana's open primary election law appears to be headed to trial.

U.S. District Judge Brian Morris ruled Monday that state and county Republicans have too many unproven claims in their lawsuit to decide the case without a trial.

The judge denied the GOP's request for an order to allow only Republicans to vote in the June 2016 primary elections.

However, Morris said in his decision, the Republican plaintiffs have presented enough evidence to continue the case.

Montana law allows voters to fill out either a Republican or a Democratic ballot when voting in primary elections.

The state GOP and 10 county central committees argue the system harms their freedom to associate with their own party members and forces candidates to change their messages to attract crossover voters.