We hear a lot about populism these days. Throughout this primary season, headlines across the country have proclaimed the successes of the “populist” contenders, Bernie Sanders and Donald Trump. Without embracing the populist label, moreover, candidates in both parties had already adopted populist tactics by branding their opponents as tools of the “establishment.”

But what is populism, anyway? There is no easy answer, for “populism” describes a political style more than a specific set of ideas or policies, and most …