Maine is similar to many Southern states except the opposite: it's not a "Blue state" in the way that Rhode Island and Washington and Oregon and Massachusetts are ... a plurality of voters are Independents, and there aren't many more Democrats than there are Republicans. If the GOP weren't seen as catering to the Religious Right/the South (granted, a very tough situation to change...), then Maine would be a swing state, IMO.
Just as many Southerners who call themselves Democrats vote Republican at the national level, many former Republicans (who are now Independents) up here would vote for what they'd consider a REAL Republican (a.k.a. a moderate) in a heartbeat, but they're very turned off by the influence cultural conservatism has had on the party. As long as the GOP doesn't reach toward the center on social issues, Maine will likely stay Democratic, but it's not out of the question that it becomes a swing state after a change in the GOP, and I wouldn't put it in the "not going to be a swing state at all in the next 20 years" category.
Another factor is transplants. To this day, I doubt you'd find a born and raised Vermonter over the age of 40 who'd proudly call themselves a Democrat ... but SO many Massachusettes and New York and Connecticut people have moved there, and the state has gone off the deep end. Maine has experienced a mini verison of that. Crazy what's happened to New England AND my party. It's my living nightmare!
Maybe that's true to an extent. Certainly, it's true that a moderate Republican would be more successful in New England.
But, let me propose an alternate theory. The strength of the Republican Party is a religious and racist panic backlash. Places like Maine, New Hampshire and Vermont are relatively irreligious, so that's out the window. The other problem is that poor people in rural New England are not scary black/Hispanic people. If middle class people don't view the poor as immoral, lazy and as the "other," not truly part of their community, egalitarian Democratic party values are more appealing.