The thing drivings the GOP poll advantage in Maine are the same we're seeing everywhere: a unified Republican base, a lack of interest from Democratic voters, a strong GOP lean with independents, and the specter of an unpopular Democratic President and Governor hanging over the heads of the party's candidates.
Stop me if you've heard this one before:
The same depressed Democratic interest that helped sink the party in the Massachusetts Senate election, as well as last fall's races for Governor in New Jersey and Virginia, is showing itself in Maine. We find those likely to vote in the state supported Barack Obama by a 7 point margin in 2008, in contrast to his actual 17 point victory there. That enthusiasm gap contributes a lot to the margin of LePage's lead but make no mistake- with him winning 76% of McCain voters and Mitchell at only 52% with Obama's he'd be in the lead even if the electorate looked the same as in 2008.
UPDATE: The DGA is up with a TV spot hitting LePage on his environmental protection policies (or lack thereof).