Governor Baker is not losing in 2018. Honestly Massachusetts' gubernatorial race doesn't matter one lick for Democrats. Baker is so moderate I'd nearly consider him a moderate Democrat. Democrat's control our state legislatures by massive super majority, and all nine of our congressional districts are controlled by Democrats, so it isn't like he can actually affect our redistricting in any way. Only way this seat could become competitive is if Kerry runs and I doubt he will.
Sentences like this infuriate me, LOL. There is no objective point where you "become" the other party because you have passed some ideological threshold. It SAYS SOMETHING about Charlie Baker that he CHOSE to run as a member of the Republican Party no matter what his views are. There would still be a fundamental difference between a Republican whose views were literally all left-of-center and a Democrat whose views were literally all right-of-center, and the former - at least in some way - would be effectively to the right of the latter, at least in the sense that he/she would believe in a basic enough principle of the GOP that he/she chose to use it as the mechanism to achieve his/her views, and vice versa for the Democrat. Joe Manchin would have several views to the right of Charlie Baker, but Charlie Baker shares the basic worldview of a Republican - he rejects the tenant of being a Democrat (that society should actively be shaped to help the less fortunate, even at the expense of more fortunate people), and Joe Manchin resoundingly accepts that tenant.
Charlie Baker is a standard New England Republican, which, yes, does place him to the left of most Republicans. However, it doesn't change the fact that he's a Republican and likely always will be. He's moderate, of course, but he represents the basic interests and motives of every Republican.