Montana’s Supreme Court on Tuesday blocked the Republican-controlled state legislature from putting forward a ballot measure this fall that would make major changes to the state’s primary system, a victory for Democrats, minor parties and labor unions who had sought to block the initiative.

The state legislature earlier this year voted to send a referendum to the ballot that would have created a “top-two primary system.” If the measure had passed, Montana candidates would run together in primary elections, regardless of party affiliation. The top two finishers would then meet in a November showdown.

Current state law allows political parties to hold separate primaries, open only to voters registered as members of those parties. The top vote-getter in each primary advances to the general election, meaning November ballots can include candidates from Republican, Democratic, Libertarian and Green Parties, along with independents and other minor parties.

The high court said the state legislature’s ballot title did not conform with state law, which limits titles to 100 words or less. The proposed top-two primary measure lists about 100 laws that would be amended, along with 11 laws that would be repealed, far exceeding the 100-word limit. The court also said the ballot language was “complicated and confusing,” the Billings Gazette reported.

Montana Attorney General Tim Fox (R) had argued the referendum’s language was legally sufficient to make the ballot. But after the Supreme Court’s ruling, a spokesman for Fox’s office said the legislature would have to alter the rules for submitting initiatives to voters.

The top-two primary system would be nearly identical to those Washington state and California use to pick their candidates, and broadly similar to Louisiana’s system. In Washington and California, the top-two primary systems replaced open primaries, under which voters could cast ballots for candidates across party lines in primary elections.

A decade ago, the U.S. Supreme Court struck down open primaries, calling them violations of political parties’ First Amendment right of association.

Montana’s teachers unions, the state AFL-CIO, AFSCME and the public employees union all joined the suit against the legislature’s proposal. Minor parties argued they would be left off November ballots if the new rules passed muster with voters.

The ruling is also a victory — even if a temporary one — for Democrats, who have won elections with less than 50 percent of the vote thanks in part to Libertarian candidates who siphon votes away from Republican nominees. Sen. Jon Tester (D) has won twice, in 2006 and 2012, with 49 percent and 48.5 percent of the vote; in both elections, vote totals for the Republican and Libertarian nominees combined to make up more than 50 percent of the vote.

The proposal passed the legislature in 2013 on narrow party-line votes, with Democrats pounding their desks to protest the Republican-led bill. The legislature is likely to go back to the drawing board when they meet again in 2015, either to change the law governing ballot measures or to try to rewrite the ballot title.