On May 4, the Maine Republican Party amended its bylaws to say that the party chooses to nominate by old-fashioned plurality primary elections. The same day, it filed a federal lawsuit, saying it has a freedom of association right to nominate according to its own desires, and that it desires not to use ranked-choice voting. Maine Republican Party v Dunlap, 1:18cv-179.
If this lawsuit were to win, then logically the Maine Green Party and the Maine Libertarian Party could also insist that they prefer to nominate by convention. Letting small qualified parties nominate by convention would be a huge advantage to those parties. Under current law, the primary petitions are so difficult, neither party is able to run more than a handful of candidates. This year, there are only about six Maine Greens running for a partisan office (all for legislature), and only one Libertarian (also legislature).
The case is assigned to Judge Jon David Levy, an Obama appointee. The Republican Party will face a severe problem getting injunctive relief, because the lawsuit and the bylaws change were made so close to the June 12 primary. But eventually the party might win declaratory relief. Even if the party did obtain injunctive relief, ranked choice voting would still go ahead for the other three qualified parties, Democratic, Green, and Libertarian.