Last April, partisanship reached new extremes in the Montana Legislature. Democratic lawmakers, shouting and pounding their desks, drowned out the Republican majority’s attempts to read Senate Bill 408. Gov. Steve Bullock, a Democrat, called the partisan warfare “worse than Washington, D.C.”

The bill, which passed on a party-line vote, sent to the November 2014 ballot a referendum that will let Montanans choose to replace party-based primaries with a top-two system: Rather than advancing the Republican, Democratic and third-party primary winners to general elections, top-two systems require all candidates to face off in a single primary. The two most-popular candidates advance, regardless of party. (Even though Bullock opposed the measure, referendum bills don’t need to be signed by the governor in Montana.)

Washington state implemented the system in 2008, as did our state of California in 2012. Advocates say the old system favors extremists and contributes to polarization. Political observers disagree on whether the reforms have helped.

But one result is undeniable: Top-two has banished minor parties, like Libertarians and Greens, from general-election ballots.

“They’re screwed,” says Todd Donovan, a Western Washington University political scientist. That prospect seems to be exactly what’s motivating Montana Republicans, who blame Libertarian spoilers for their recent narrow losses to Democrats.

Many politicos see traditional primaries, which generally allow only registered Democrats or Republicans to vote, as partly to blame for congressional dysfunction. Both parties have painstakingly redrawn legislative districts to make them safe bets. Primaries in some of these reliably red or blue districts have become more decisive than general elections, forcing candidates to court the voters that turn out for them—often the parties’ most right- or left-wing members.

This puts centrists at a disadvantage. Many “establishment” House Republicans, for instance, took a back seat to their Tea Party colleagues in last year’s government shutdown, fearing that any compromise with Democrats would provoke primary challenges from conservative ideologues. The resulting crises have prompted more calls for primary reform, and rebellion among conservative allies dissatisfied with the gridlock. The U.S. Chamber of Commerce, for instance, is throwing its weight behind old-guard Republicans like Idaho Rep. Mike Simpson, who is facing a Tea Party primary opponent. A pro-business political group is doing the same in Montana.

California state legislators, unable to compromise over taxes and spending, also created regular budget crises. In response, Republican Gov. Arnold Schwarzenegger helped lead a successful push for reforms, including the top-two primary system. It would promote moderation, proponents believed, because candidates would have to appeal not only to their ideological base, but to voters of all political leanings.

For the top-two system to work, however, centrists have to vote. Turnout for California’s 2012 primaries slumped to its lowest ever, while Washington’s dropped to its third-lowest. That may be why California politicians are no more aligned with the average voter than they were before the change, according to two survey-based studies. And Washington’s Legislature was already fairly moderate, says Donovan, who has seen little evidence of political change.

Advocates say top-two just needs more time. Still, they believe it’s already helping: Last fall, former Republican strategist Dan Schnur told The New York Times, “You see Republicans voting for immigration reform; you see Democrats voting for streamlining environmental regulations.”

In any case, the new system has undoubtedly further marginalized minority parties. Third-party congressional candidates appeared regularly on Washington’s ballot before 2008; only one has done so since. In California, where more than 20 percent of voters are registered independents, the only third-party candidate on the 2012 general-election ballot was Libertarian presidential candidate Gary Johnson.

While sidelining minority parties was never the goal in Washington and California, it appears to be driving the GOP’s push for a top-two system in Montana. In 2006, incumbent U.S. Sen. Conrad Burns, a Republican, lost to Democrat Jon Tester by only 3,562 votes. A Libertarian took about 10,000 votes. In 2012, many think Libertarian candidate Dan Cox, with help from liberal dark money, siphoned votes from Republican Denny Rehberg, enabling Tester’s second victory. Cox drew 31,892 votes—13,820 more than Rehberg lost by. A Libertarian took 4 percent of votes in the 2012 governor’s race, which Democrat Steve Bullock won by just 2 points.

But a top-two system may not guarantee future GOP victories, says Richard Winger, a California-based ballot-access analyst. The theory that Libertarians spoiled these races assumes that the votes they draw would otherwise go to Republicans. That’s not always true. When a Libertarian won 6.5 percent of the vote in Virginia’s last governor’s race, exit polls showed that those voters were largely pro-choice, and some favored the Democrat over the Republican. Either way, Montana Libertarians anticipate extinction if the state adopts top-two. “You have to wonder if (the Republicans) actually believe in the free market, because they are trying to use the forces of government to their ends,” says Montana Libertarian Party chair Mike Fellows.

Not everyone is worried about the potential demise of third parties. Montana political scientist Jim Lopach thinks that top-two would have a net benefit for the state if it had a moderating influence. On the other hand, general elections are “where people discuss what they want in an office, and what they want in public policy,” says Andrew Spencer, an attorney with FairVote, a voter advocacy group. A third-party presence can help shape the debate, and even policy. Independent Ross Perot, who campaigned for president in 1992 on fiscal prudence and took nearly 19 percent of the popular vote, is credited with influencing Bill Clinton’s effort to balance the federal budget.

“If you opened up a good burger restaurant, more people are still going to eat at Burger King and McDonald’s,” says Montana Libertarian Dan Cox. “It takes time to get your share of the marketplace.”

However, in Montana, as in California, time may be running out.

This article originally appeared in High Country News.