On May 17, attorneys for Maine filed this 24-page brief in Maine Republican Party v Dunlap, 1:18cv-179. The lawsuit had been filed earlier this month by the Republican Party, which argued that its freedom of association rights means that the party ought to be free to avoid using ranked choice voting.
The Maine government brief notes that the Republican Party uses a form of ranked choice voting at state conventions, to choose party officers. See page 10. The government brief also has a scholarly footnote 7, mentioning instances from approximately 100 years ago when states required parties to use ranked choice voting in their primaries.