In the U.S., enormous battles have been fought between establishment political parties and outside challengers labeled as “populists” — including Republican Donald Trump, former Democrat Robert F. Kennedy, Jr. and socialist Bernie Sanders. Populism is being fueled by massive voter frustration. And it’s changing the face of politics around the world.

Warsaw, Poland, in Eastern Europe, is ground zero for a familiar-sounding theme: voters rejecting establishment leaders and electing a different breed. I recently spoke with Polish President Andrzej Duda, who’s frequently derided as a populist.

“If I listened to my electorate, the people who were talking to me at various rallies and meetings, and if I delivered on what I promised, does that make me a populist?” he asked. “I don’t think so. I think I’m a democratic politician who honestly delivers on his political mandate. I listen to what they say, I make my commitments, and then I submit myself to their assessment.”

Eight years of his Law and Justice Party’s conservative rule have brought tough policies against illegal immigration and criticism of the European Union.

I asked him why populism is seen as a negative thing, since it generally refers to a popular candidate who responds to popular sentiment.

“In Poland, if we speak of a populist politician, a populist — this sounds negative,” he said. “A populist is someone who really relies on the most base instincts of the electorate and employs them in order to obtain some political benefits. And if so, I am not a populist. I think I’m an earnest politician who takes his role seriously.”

The growing populist sentiment is influencing politics around the world — threatening parties that called the shots for decades, leaving many citizens feeling unheard.

“They have been caricatured very effectively by their opponents — that populism means, you know, neo-Nazis almost, or people stirring up demons from the deep and appealing to the lowest common denominator in politics,” said David Cowling, a political analyst in Great Britain, a major center where populism is upending politics. “All of which, in my experience, is just so much rubbish, really.”

Cowling described what he calls “a bit of a political earthquake” in the last 20 years.

“People have decided that there are alternatives they want to investigate because the established parties that have dominated most of the post-war period are seen as failures,” he said.

In Poland, President Duda’s Law and Justice Party won more votes than any other party in last year’s elections. However, three main opposition parties formed a bigger coalition, won a majority in Parliament, and appointed a prime minister who promised to unravel what Duda and his party had done.

Here in the U.S., while Trump is widely called a populist, many analysts say Harris and her policies also fall under populist categories.

I asked Aaron Korewa, director of the Atlantic Council’s Warsaw, Poland office, whether he sees populism as part of an evolving system, or as a danger.

“Well, it depends a little bit,” he said. “A lot of the things that people worried about that Donald Trump was gonna do, I mean, we know that Donald Trump has already been in power for four years. I mean, it’s not like we’ve seen a complete, you know, flipping of the table or whatever you want to call it in the United States. And that’s usually not what happens when these parties come to power. So I do think that, in many ways, they tend to sort of become part of the system, eventually. And whether you like it or not, they do pick up on a sentiment that exists in society.”

“Full Measure with Sharyl Attkisson” airs at 10 a.m. Sunday, WJLA (Channel 7) and WBFF (Channel 45).