I'm going to disagree with everyone here...
Susan Collins is pretty moderate, but she has still stuck with the GOP on most major issues, specifically Obamacare. She also has a more conservative VP, so I think she'll be able to do reasonably well with Republican turnout while also attracting quite a few moderates. Joe Manchin has frustrated quite a few Democrats, a lot more than Collins for Republicans, as he has opposed Democrats on quite a few major issues. Beshear does nothing to solve that problem.
I think Collins wins this by a moderate margin, and stands a decent chance at winning the female vote with her pro-choice stance. Low turnout all-around, though.
Rural vote significantly narrows from 2012, but Collins wins the suburban vote and makes gains in the urban vote. Pennsylvania would be a fun state to watch in this election.