From Richard Winger at Ballot Access News:
Maine requires a group that wishes to become a qualified party to obtain 5,000 registered members. This requirement was created in 2013, and replaced the old, very difficult 5% petition that had been in the law starting in 1976. The Maine Libertarian Party has been carrying on a voter registration drive for the last six months, and now has over 5,000 registrations, although they have not all yet been submitted to election officials. The drive is continuing with a goal of 6,000, for safety.
The last time the Libertarian Party was a qualified party in Maine was 1991-1992. Currently the Green Party is ballot-qualified in Maine. The Reform Party was also a ballot-qualified party in Maine in the 1990’s, but before that Maine hadn’t had any ballot-qualified parties other than the Republican and Democratic Parties since the 1910’s decade.