Last fall, Maine voters passed an experiment in voting that no state has ever before tried: ranked-choice voting. It’s an experiment some say could change the national calculus against third parties, as I’ll explain below. But the state’s Republican-led Senate asked Maine’s Supreme Court to rule on the system — and the court recently issued an advisory ruling that ranked-choice violates the state constitution.

So why would anyone be interested in ranked-choice voting — and why are Maine’s Republicans fighting it?

Okay, what’s ranked-choice voting, and why would it be unconstitutional?

A voter ranks candidates from one to six. If no candidate gets a majority of first-choice votes, the last-placed candidate is dropped. Ballots for the dropped candidate move to the next-ranked person on each ballot. The process repeats until one person has a majority. Slate’s Henry Grabar explains the system this way:

Also known as “instant runoff” voting, the system makes third-party candidates more viable by eliminating the spoiler effect. Voters who picked the candidate with the fewest first-place votes have their votes instantly reallocated until one candidate has a majority.

But that word “majority” is the problem. According to Maine’s Supreme Court, the system violates a state constitutional rule that winners only need the most votes — a plurality — and not a majority. Ranked-choice requires a majority. If the candidate who gets the most first-choice votes — a plurality — has fewer than 50 percent plus one, ranked-choice doesn’t award him or her the office. Rather, it drops the last-place candidate and keeps adding up the newly available votes.

No Maine governor has won a majority of votes since 1998. One or more independent candidates have held the balance in each of those elections.

Why are Republicans fighting ranked-choice voting?

The answer is that Maine Republicans cannot now win a statewide, ranked-voting majority. To understand why, we need to examine how swing voters will use second-place rankings. Which of the two major parties would they rank above the other? To figure this out, I probed the likely voting behavior of registered independents. I assumed that a ranked-choice voter would give the highest ranking to a candidate closest to their own position; second-place ranking to the next-closest candidate; and so on.

Using data from the Cooperative Congressional Election Study, we can measure how close a registered independent says he or she is to various candidates and national parties. CCES respondents are asked to place themselves along a scale from liberal to conservative. Then they are asked to do the same thing for the Democratic Party, Republican Party and a range of state and national political figures.

Here’s what I found when I analyzed CCES data for Maine voters from 2012, 2014 and 2016.

In 2016, Maine’s registered independents said they were closer to named Democratic candidates and to the Democratic Party than to Republicans. Perhaps more notably, independents said they were closer to the Democratic Party than they were to sitting Republican Gov. Paul LePage, who won his 2010 and 2014 elections with less than a majority. In 2014, however, independents said they were closer to LePage and the Republican Party than to most Democrats.

That’s why Democrats saw 2016 (but not 2014) as a good year to back ranked-choice voting — and Republicans a good year to oppose it.

So what comes now, after the Maine Supreme Court ruling?

The court said the state legislature could either repeal the new ranking system and go back to winner-takes-all voting, or amend the state constitution to enable ranked-choice. In which direction will the legislature move?

The recent court ruling came from a GOP test case. Republicans wanted to learn what the high court would do if a ranked-choice majority winner did not also have the most first-choice votes (a plurality). Now they know.

If Republicans challenge the result of a ranked-choice vote count, the court may invalidate that election. It might even rule that the person with the most first-choice votes is the winner. If recent gubernatorial elections give any indication, that will be a Republican.

In return, Democrats hope to amend the state constitution, replacing the troublesome plurality rule with language that mandates majorities.

But getting there won’t be easy. Any constitutional amendment requires a two-thirds vote in each state house before it can go to voters, and Republicans say they won’t go along. The parties have filed competing bills. A Democratic bill would change the constitution, and Republicans’ proposal would repeal ranked-choice. Democrats control the state legislature’s lower chamber, and Republicans have a one-vote Senate majority.

Republicans’ Senate majority also means they can block ranked-choice implementation — even without voting to override the 2016 referendum. Before any election uses a new system, poll workers need to be trained. Voting equipment needs to be upgraded to be able to capture rankings. All of that costs money, as the high court noted in its opinion. And Republicans would have to authorize that spending.

In other words, the fate of Maine’s ranked-choice voting depends on these two things

First, which party controls the lawmaking bodies needed to implement ranked-choice voting? And does that party think that independents will rank its candidates higher or lower than they rank the opposing party?

Right now, while public opinion favors the Democrats, control of state government does not. Ranked-choice voting will be stymied, unless Democrats persuade some Republican lawmakers to cross to their side.

Midterm elections in 2018 could shift that math. On one hand, Democrats might win back state Senate control while holding control of the House — although it’s not very likely to win a two-thirds supermajority in both chambers. On the other hand, independents could begin favoring Republicans over Democrats. That’s what my analysis showed for 2014, the most recent midterm year: Independents preferred Republicans to Democrats.

A midterm shift in independent voters’ preferences could warm the GOP to ranked voting. It also could cool off the Democrats who backed ranked-choice voting in the first place.

So will Maine voters ever get to rank their choices? We’ll have to wait and see.

Jack Santucci is a political scientist who studies alternative voting rules in the United States. Follow him on Twitter @jacksantucci.