Well, in the defense of Kentucky, Bevin himself is who made this a race with making some really bad comments at times and damaging his own personal popularity - otherwise I really don’t think this would even be a contest. The air war has effectively refocused the race on policy instead of personality which has damaged Beshear and strengthened Bevin. The Democrats had been solidly in control of the state House since the 1920s until the 2016 election and it flipped big time to a Republican supermajority and it held on in 2018 - I believe that is here to stay.
From the 19th century until the 1990s, no Governor could run for reelection in KY (unless they succeeded to the office after the start of a term). There has never been a Republican reelected Governor. Only once in state history has a Republican succeeded another, and that was in 1899 when William Sylvester Taylor did so, only to have the election stolen by a Democrat Board of Elections and handed to the loser, William Goebel (who himself had just created said board, which was massively unpopular). That decision just about triggered a civil war in the state and Goebel was promptly assassinated on his 4th DAY in office in Feb 1900.