If you stay home you will do one vote worth of damage.
If you vote for the Democrat, you will do two votes worth of damage.
If you've made up your mind to hurt McConnell, why do it half way?
Were I in Kentucky, I would, for the first time in my life, vote for the Democrat. I would regard it as a duty to do so.
I would not vote Democrat any more than I would vote for McConnell.