McCONhell knows some Repug candidates suck

In particular, Vance, Walker and Oz. He's made it quite clear in more general public statements. His motivation though is purely about winning back a Senate majority. And these candidates are going to prevent that. 

McCON knows though that he must keep quiet about the underlying reasons. That is, this anti-democracy, pro-fascist takeover of his Party from top to bottom. But he knows it to be true given his criticism of how that dynamic has led to their primary victories. So he is just as much responsible for that fascist takeover because he won't openly admit and fight it. Instead he chooses to accept it to keep those voters. It's a must-win to retain power and democracy be damned in the process. 

 PS: Also see Krystal discussing it starting at 2:35 in this video.