PORTLAND, Maine — Maine’s Republican governor challenged the validity of Tuesday’s primary as residents used ranked-choice voting for the first time to choose gubernatorial candidates, while also deciding whether to keep the system in place for elections in November.

Republican Gov. Paul LePage, a long-time opponent of ranked voting, called the election overhaul the “most horrific thing in the world.” He threatened not to certify Tuesday’s election results, but Maine’s top election official quickly said that the governor can’t stop primary election results from moving forward.

The system, which lets voters rank candidates from first to last, is facing its biggest test yet in a statewide primary election. It is used in 11 local jurisdictions across the country.

Voters are using the system to sort through a crowded gubernatorial field that includes 11 candidates — seven Democrats and four Republicans who are looking to replace the term-limited LePage. It’s also being used in the Democratic primary in the 2nd Congressional District, which covers most of Maine, and in the GOP primary in House District 75 west of the capital, Augusta.

Democratic Secretary of State Matt Dunlap said the governor could refuse to sign a proclamation of the results but that still wouldn’t stop the nominations from taking effect.

“He can bluster,” Dunlap said.

Also on the ballot is a “people’s veto” question aimed at nullifying a legislative delay so that ranked-choice voting can be used in federal elections in Maine in November. Dunlap said LePage would need to certify the results of the referendum.

Many voters on Tuesday had a firm grasp of the ballot.

“It’s pretty easy to do, despite the negative publicity. I can count to seven, and they can do the math on the other end,” said David Kuchta, of Portland, referring to the number of candidate he ranked in the Democratic gubernatorial primary.

The voting system allows voters to rank their candidate preferences from first to last on the ballot. If there’s no majority winner, then there are additional voting rounds.

Supporters say the system ensures a majority winner, eliminates the impact of spoilers and encourages civility in campaigns. Opponents have charged that it’s confusing and doesn’t square with the Maine Constitution.

The system also has faced legal challenges. Maine’s highest court cleared the way in April for voters to use the ranking system.

Ranked-choice supporters gathered enough signatures to get the people’s veto question on the ballot after state lawmakers voted to delay ranked-choice voting until 2021, when it would be repealed unless allowed by a constitutional amendment.

The voting system can be used only in statewide primaries and in federal general elections in Maine. It can’t be used for legislative and gubernatorial general elections because of concerns it runs afoul of the Maine Constitution.

On Tuesday, several Democratic voters pointed to the election of LePage in 2010 for supporting the voting system. LePage won without a majority in a multi-candidate race in which Democrats torn between two other candidates split their votes.

A few were surprised to learn that new voting system won’t apply to that governor’s race in November, even if residents agree to carry it forward.

“Things take time, unfortunately. It’d be nice if thing went faster,” said Carmine Terracciano of Portland. “You gotta start somewhere.”

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Associated Press writers David Sharp and Marina Villeneuve in Portland contributed to this report.

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