…this is right now:
And do you know what keeps your name or the name of some political enemy of the president or the name of all of Congress or the Senate off that list?
Custom. Relative economic stability. Lack of opportunity to weed out military and law enforcement officials who would revolt rather than stand by while the Executive seized power.
But a nation which is quiescent while the Executive grabs this kind of unaccountable power to kill whoever he likes is a nation that is simply awaiting the right combination of circumstances to allow a de jure tyranny to become a de facto one.