McCain, Collins, Murkowski, Capito, Portman, and a few others are liberals at heart.
McCain is dying and will not run again. He's free to be his liberal heart.
Capito could be primaried and in this current climate in west Virginia, it would probably yield a more conservative repub senator.
Collins is dangerous. She's the most liberal in the repub senate, moreso than many democrats. She could easily switch parties, and a democrat will likely win her seat in liberal Maine if she chooses to retire. Primarying her would turn the seat democrat, but at least you'd know the real score. On the other hand, if it flips control of the senate, that is a big loss.
Murkowski is in a state that is not a sure win for repubs in congressional races.
Portman is also in a state that is a 50/50 proposition. Portman votes conservative when he has cover, like what John McCain provides. OTOH, his constituency is conservative, so he must carefully pick his liberal votes. As a congressman he voted and talked extremely conservative in OH 02 district. The Bushes either converted him or allowed him to come out of the closet a bit. Bottom line, he isn't trustworthy.
Losing these seats would flip the Senate to democrat control. That would ruin appointments and minor legislative advance.
McConnell knows all this and walks on eggshells to keep his power.
That's why I tell people, here in Ohio, we have 1.5 liberals in the Senate.