The primaries for this election were the first in Maine to be conducted with ranked choice voting (RCV), as opposed to a simple plurality, after voters passed a citizen referendum approving the change in 2016.

[5] An advisory opinion by the Maine Supreme Judicial Court held that RCV would be unconstitutional for general elections for governor and the state legislature.

[6][7] Backers of a "people's veto" turned in enough signatures to suspend this law until a June referendum vote, which restored RCV for future primary and congressional elections.

[8] Governor Paul LePage threatened not to certify the results of the primary elections, saying he would "leave it up to the courts to decide.

Former state senator and former mayor of Lewiston and Auburn John Jenkins and perennial candidate Kenneth Capron ran write-in campaigns.

Mills was also the first gubernatorial candidate to win at least 50% of the vote since Angus King in 1998, and the first non-incumbent to do so since Kenneth M. Curtis in 1966.

LePage did not win a majority of the vote either time (receiving 37.6% in a crowded four-way race in 2010 and 48.2% in a three-way race in 2014), with Democrats accusing independent candidate Eliot Cutler of splitting the anti-LePage vote in both instances, though Cutler finished closer to LePage than Democratic candidate Libby Mitchell in the 2010 election.

[13] Maine's history of governors elected without majorities, including LePage, was one impetus for the citizen's referendum to implement ranked choice voting.

Though ranked-choice voting was approved by voters in a 2016 referendum, the Maine Legislature voted to delay and potentially repeal RCV for all elections after an advisory opinion by the Maine Supreme Judicial Court ruled it unconstitutional for general elections for state offices.

RCV supporters succeeded in a people's veto effort to prevent the delay, which suspends it until a June 2018 referendum vote.

[21] The Maine Republican Party filed a federal lawsuit in U.S. District Court in Bangor on May 4, 2018, seeking to bar the use of RCV for its own primary on the grounds that requiring the party to use it violates its First Amendment rights to choose its nominee as it sees fit.

On October 29, in a press conference at the main branch of the Portland Public Library, Caron dropped out of the race and endorsed Mills.