I think McConnell sees his mission now is to keep another conservative from getting elected, so to that end, he MUST BEAT BEVIN! If he loses in November, so be it, but at least it would be to a democrat, which is the natural order of things.
These people hate and resent the conservatives for taking out first Arlen Specter, then Bob Bennett, then preventing Mike Castle and Charlie Crist from becoming nominees and thus senators, then making Lisa Murkowski fight for her political life, then taking out Richard Lugar and then defeating David Dewhurst.
These old guard types literally HATE conservatives and will do whatever it takes to defeat them.
-PJ