"What He Has Tapped into Is What the Founders Most Feared"

"This phenomenon has arisen in other democratic and quasi-democratic countries over the past century, and it has generally been called 'fascism.' Fascist movements, too, had no coherent ideology, no clear set of prescriptions for what ailed society. 'National socialism' was a bundle of contradictions, united chiefly by what, and who, it opposed; fascism in Italy was anti-liberal, anti-democratic, anti-Marxist, anti-capitalist and anti-clerical. Successful fascism was not about policies but about the strongman, the leader (Il Duce, Der Führer), in whom could be entrusted the fate of the nation. Whatever the problem, he could fix it. Whatever the threat, internal or external, he could vanquish it, and it was unnecessary for him to explain how. Today, there is Putinism, which also has nothing to do with belief or policy but is about the tough man who single-handedly defends his people against all threats, foreign and domestic.
"To understand how such movements take over a democracy, one only has to watch the Republican Party today."

Robert Kagan in a 2016 Washington Post article defines the threat from Donald Trump.