What with this and Angus King, does Maine have a thing for independents that I just haven't heard of?
Yes. Eliot Cutler got over one-third of the vote and came in second in 2010. There was also an independent, Barbara Merrill, who got over 20% in the 2006 governor's race. Maine is the only state to have had more than one independent governor (King, and Jim Longley in the 1970s). Maine was also Ross Perot's best state in 1992. The Maine Green Party has consistently been the best-performing state Green Party in the country as well, having elected a State Representative in Portland for a while in the early 2000s and routinely getting close to 10% of the vote in gubernatorial races (though they didn't run a candidate in 2010 and endorsed Cutler instead).
It is safe to say that Maine is the most favorable state in the Union for non-aligned candidates.
