Go back in history.  Even recent history.  Suppose Obama were inclined to pursue the prejudices or viewpoints many ascribed to him with the power the Trumpians now claim.  Pick something Obama didn't appear to like, or COULD draw a line between conduct and risk to national security.  Maybe even a line between US national security and historical proclivity for violent anarchist or political behavior.  Suppose Obama focused on Timothy McVeigh, and those like minded.  Under Trumpian argument, Obama could have declared national security risk due to McVeigh's terrorist tendencies, and then excluded all who shared McVeigh's national orgin(s).  Do we want to empower ANY US President to do that?  What if a Muslim gains the Presidency?  What if Steve Bannon actually becomes the elected President?