From Richard Winger at Ballot Access News:
Maine requires a group that wishes to be a ballot-qualified party to have at least 5,000 registered members by December 1 of the odd year before an election year. The only group that tried to use this procedure for 2016 is the Libertarian Party, which has been registering members all during 2015. The party has submitted 6,482 voter registration cards. The state will say on or before December 8 whether the party has enough registered members.
This registration procedure in Maine has only existed since 2013. The former procedure was very harsh; the old procedure required a petition of 5% of the last gubernatorial vote, and all the signers had to be registered voters who were not members of a qualified party. That procedure was so severe, it was only used twice, by the Reform Party in 1996 and Americans Elect in 2012. One reason the 2013 session of the legislature changed the procedure was because the Americans Elect experience showed how massively difficult the old procedure was. The old procedure had been created in 1976.
The Green Party is also ballot-qualified in Maine. It got that status by using the independent candidate procedure to place its gubernatorial nominee on the November ballot and then polling over 5% for Governor. The first time the Green Party did that was in 1994, when Jonathan Carter got 6.4%.
The only other time the Libertarian Party was a ballot-qualified party in Maine was in 1991 and 1992. It got that status by having an independent candidate poll over 5% for Governor. The independent, Andrew Adam, after the election was over, told the Secretary of State to assign his votes to the Libertarian Party.