IMHO, it's rather unbecoming for McConnell to take sides in the Mississippi run-off, but he can if he wants to.
The Kentucky Republican primary is settled, period, and by a rather significant amount in favor of McConnell, like it or not. Since Kentucky is not a reliably concervative state, a write-in campaign for Bevin or a "no vote" protest against McConnell would most likely result in whoever the Democrat nominee is being elected. This doesn't seem helpful.
OTOH, Mississippi IS a reliably conservative state, and if McDaniel wins the run-off, he will very likely be elected in the general election, despite any McConnell supporters withholding support or protest voting for the Democrat (which I doubt they would do, but you never know!). This IS helpful, and personally, I hope that's what happens!
Attributed to W. F. Buckley (paraphrased): "I will always vote for the most conservative candidate that can win."
In a state-wide election, it seems prudent to consider that. It's perhaps better to hold your nose than cut it off...JMHO.
What happens when you have no conservative? McConnell is as reliable as McStain to stab you in the back. I know several conservatives who are sitting out the November election for senate. Conservatives who have voted Republican since Goldwater.
McConnell said he wanted to punch conservatives and the tea party in the mouth and did so. Now he expects us to buy him and his buddy Thad lunch and another term as senator come November? Up his and the GOPE!
I guess Mitch forgot the 11th Commandment——again.