"As goes Maine, so goes the nation." This was the rule of thumb for political prognosticators for decades. Maine is a quirky state, and for years it held its elections in September. This gave an early view of the mood of the country in the time before reliable public opinion polling. But make no mistake, for years the only question concerned the degree of victory the Republican candidate would earn in Maine. Until 1954, when Ed Muskie won, Republicans had lost only five gubernatorial elections -- at a time when those elections were held every two years. Since the Civil War, Maine Democrats have sent only a handful of senators to Washington; most have been Republicans, although they have been cut from a decidedly moderate, or even liberal, cloth.
Maine experienced a bit of a tsunami in 2010, electing a fiscally and socially conservative governor in Paul LePage, and giving him a Republican legislature to work with; LePage was re-elected in 2014, although the party only controls the state Senate today. This led to some speculation that Olympia Snowe would be vulnerable to a challenge in the Republican primary. She instead retired, throwing Maine politics into turmoil.
Both parties had crowded primaries. Republicans nominated Secretary of State Charlie Summers, while Democrats opted for state Sen. Cynthia Dill. But the victor was former governor Angus King, who served in the late 1990s and early 2000s. King was elected as an Independent, although he opted to caucus with the Democrats. This time King has avoided a challenge from either top tier Republicans or Democrats, and is in good shape for re-election.