Bevin wound up uniquely unpopular because he, as a state politician, got actual blame for issues like fucking over teachers that congresspeople usually get away with by doing more indirectly. Bevin also got none of the credit for things like anti-choice judges that make R senators popular with their base. Then, Bevin also injected himself into every second of everything like a wannabe Trump and acted like a crass asshole that it was impossible for his potential voters to ignore. He layered a bonus Trumpian effect atop the normal trump effect and made tons of people who otherwise voted straight R vote for beshear.
McConnell will get more credit for appointing religious extremists to lifetime judgeships, less blame for ruining the lives of his constituents, and will not constantly be on the news making incredibly dumb unforced errors that annoy the fuck out of people. He's probably still safe unless our candidate turns out to be an absolute campaign beast and the economy completely dies in 6 months.
Watch this space.