Donald Trump and his populist, nationalist counterparts in Europe are often portrayed as a threat to democracy. Their supporters argue that establishment politicians and technocrats are an even bigger threat because they listen only to the special interests that feed them. Could it be, then, that democracy in its current American and European forms is a threat to itself? There’s evidence that it may be.

Liberal democracy in the 21st century has become far more the norm than the exceptional form of rule. Since the 19th century, liberal democracies have grown in number, at first gradually and fitfully, then exponentially since the 1980s. So dramatic was the global rise in democratic rule by the early 1990s, it was thought to…