Trump himself has already made it clear that being illegal is not the moral line in the sand, as evidenced by firing Flynn, when saying it is because he has lost "trust" in him.  So, "trust" is the requirement.  Trump will now have to say he trusts those who communicate with Russian spies, or fire them.  The cumulative weight of this will slowly crush him, and the timeline is short, the goal is to have it happen before the next congressional elections