AUGUSTA, Maine (BDN) — In a surprise move, Maine lawmakers passed a bill Monday extending ranked-choice voting to presidential primary and general elections, though it’s unclear if Gov. Janet Mills backs it. She could stop it from taking effect before the 2020 election.

The proposal from Senate President Troy Jackson, D-Allagash, would put Maine further in uncharted territory on ranked-choice voting. A 2016 referendum made it the first state to adopt ranked-choice voting statewide, but legislative tweaks motivated by conflicts with the state Constitution have limited its use to congressional elections and state primaries.

The Democratic-led Maine Legislature voted in June to ditch party-run presidential nominating caucuses for state-run primaries, but lawmakers left Jackson’s ranked-choice measure one Senate vote shy of approval before adjourning at the end of the regular 2019 session in June.

It was revived Monday toward the end of a one-day special session convened to address bonds proposed by Mills, a Democrat who said on Monday that she wanted the Legislature to only work on those measures and other technical fixes to Maine law.

Republicans blocked three of the four bond bills favored by Mills. Jackson’s office signaled early in the day that the ranked-choice bill would not come up, but the Senate president moved it to a hasty vote in the evening. It passed the upper chamber in a 20-12 vote without debate. All Democrats except Sen. Bill Diamond of Windham backed it, and all Republicans opposed it.

Jackson said he wanted to back “the will of the voters,” who upheld the 2016 law in another vote last year, since Mainers have “passed this twice and this is the last opportunity to do it” before the election.

Spokespeople for Mills did not respond to a question about whether she supports the bill. Republicans have opposed the measure, so a veto from the governor is likely to kill it.

It was the best hope for ranked-choice advocates to expand the method’s use in Maine in 2019, since it could be done as a simple change to law and not as a constitutional amendment, which would have required two-thirds votes in both chambers and Republican support to advance.

Its immediate impact would be in Maine’s presidential primaries March 3, when more than 20 well-known Democrats are running for the nomination to face President Donald Trump, a Republican. The office of Secretary of State Matt Dunlap, a Democrat, flagged several concerns with Jackson’s bill when it was proposed earlier this year.

They included that nominating elections are indirect — meaning they govern delegates chosen at party conventions according to party rules. Six state Democratic parties have plans to include ranked-choice voting in their processes, according to FairVote, an electoral reform group.

In ranked-choice elections with a single winner, voters choose multiple candidates in order of their preference. And if no candidate gets more than 50 percent of the total vote, the last-place finisher is eliminated, and the second choices of voters who cast ballots for that candidate are used in a retabulation. The process repeats until a winner emerges with a majority of the votes.

If the measure becomes law, Maine’s process could be modified to fit a Democratic National Committee rule setting a minimum threshold of 15 percent of votes to get delegates by state. The ranked-choice count could continue until the last remaining candidate crosses that threshold, then delegates could be allocated proportionally.