Donald Trump's presidential campaign rhetoric is, by most accounts, "populist, " but that's a broad description. Trump takes his "populism" from a particular historical tradition -- one with a baleful history in American politics.
What is populism, and what's the problem with Trump's version of it?
Simplified, populism is the notion that society consists of two classes -- the righteous but oppressed masses, and the greedy and oppressive power elites. That notion is timeless, but in modern political theory we can trace it to two French libertarians, Charles Comte and Charles Dunoyer, who correctly identified the righteous masses as "the productive class" (those who make their living through honest labor and exchange) and the greedy power elites as "the political class" (those who make their living, and accrue their power, by working for or buying the favor of the state).
Karl Marx repurposed Comte's and Dunoyer's theory and put it in harness to his nutty economic theories. Marx's righteous masses were "the workers;" his greedy power elites were "the capitalists." His proposed solutions militated in a non-libertarian direction, but he was at least clear on the relationship between the power elites and the state. The state, he said, is "the executive committee of the ruling class."
The final disposition of the ruling class is the rub with most "populist" agitators: They aim to topple the existing ruling class and replace it with another. They don't want to get rid of the power elites; they just want to BECOME the power elites. And they promise that their constituents (the righteous masses) will ascend to power with them.
Principled populism aims to end the existing class division altogether. By either limiting or liquidating government, it proposes to make the formation or existence of a "political class" impossible. In a genuine populist society, a libertarian society, honest labor and free exchange are the sole sources of wealth and power.
Trump's populism descends from an odd twist in American populism which treats the most marginalized and oppressed groups as the oppressive power elites, the middle class as the oppressed righteous masses, and a demagogue as the savior of those masses. We saw this kind of populism in the Dixiecrat rebellion of 1948, in George Wallace's independent presidential campaigns, in Nixon's "southern strategy" and in Pat Buchanan's upstart Republican and Reform Party efforts.
Trump tells Pennsylvania steel workers and Louisiana carpenters and Kansas farmers that the oppressive power elites aren't the political class (American government's taxers, regulators and subsidy eaters), but rather foreign workers crossing the border and foreign governments American politicians get "a bad deal" from.
He tells the white middle class that the power elites aren't the political class (government police terrorizing our communities), but their fellow productive class Americans (often African-Americans) who object to assault and even slaughter by those police.
He tells Americans that putting him in power will put them in power.
Don't fall for it. It's a lie. Trump's a fake populist and a run-of-the-mill (except for the really bad hair) power seeker.