The prospect that Roy Moore will be elected the next senator from Alabama despite an increasing number of allegations that he sexually assaulted and “dated” teenagers while he was in his 30s is rightly terrifying. As a result, some Senate Republicans are saying that Moore should be expelled immediately from the chamber if he wins the election so he can be replaced by an alternate. But that prospect is also potentially terrifying: It could hand the White House a way to shut down special counsel Robert S. Mueller III’s investigation. For good.