It’s still very early, but Chellie Pingree is hesitating rather than jumping into the race. Among Republicans, it seems that Secretary of State Charlie Summers is the most likely nominee. Anything can happen.
Since Maine has had a series of Anus U.S. Senators, maybe inserting the “g” in the middle will change things.
If the Dems decline to field a first tier candidate, you will know that some type of deal is afoot. But in a genuine 3 way race, the GOP has a good shot.
Don’t forget Maine went totally Republican in 2010 — governor and legislature. Anything is possible.