I don't expect that Trump will win in November but I sure think he could. I certainly don't believe that some misty American exceptionalism will forever protect us like a magic charm from demagogues or tyrants. American exceptionalism is just as likely to lead to a Trump presidency as it is to prevent one.
There is a recurring theme throughout the 2016 presidential campaign, particularly on the Republican side: "Be afraid -- be very afraid." It is enough to conclude American greatness lies in our historical rearview mirror. It is somewhat counterintuitive if one also accepts the myth of "American exceptionalism."