The Maine Green Party is ballot-qualified, and must nominate all its candidates in its own primary. It is so difficult for small qualified parties in Maine to get candidates on their own party’s primary ballot, the Green Party has never been able to get anyone on the ballot for either house of congress, in years when it was qualified. However, in 2020, David Gibson will try to run for U.S. Senate. See this story.
The Maine 2020 U.S. Senate election will use ranked choice voting. The 2018 U.S. Senate race also used ranked choice voting, but it was moot, because Angus King received a majority of first-choice votes. Thanks to the commentator for this correction.