Hugh Espey is sitting in a room strewn with protest signs, pounding the table and talking about why Americans are angry.

“We feel that the Democratic Party has sold us down the river,” explains Espey, a veteran political organizer who wears a beanie and talks like he just drank three shots of espresso. “There’s lots more like us, and it gets back to the righteous anger. Not hateful anger. Not blind-rage anger. We’re sick and tired of being messed over by the political establishment.”

Espey is the executive director of Iowa Citizens for Community Improvement Action Fund, a populist organization that has endorsed Senator Bernie Sanders. Given Espey’s perspective, the pick shouldn’t be a surprise. More than any other Democratic candidate, Sanders has channeled the outrage that a lot of voters are feeling. It’s not just anger aimed at President Donald Trump, though there’s plenty of that. It’s the frustration at years of being let down by politicians who were supposed to be on their side, of decades of financial inequality, of inaction on the issues that most affect them, from health care to climate change.

“Righteous anger” is a theme at virtually every Sanders campaign event. At a rally held on a chilly January day in the basement of Iowa City’s The Graduate Hotel, Sanders and his allies talked about the urgency of climate change. He described Australia burning and, taking care not to sound “too alarmist,” argued that what’s happening there will spread all over the planet if we fail to act.

“If we do not get our act together with a fierce sense of urgency—and let me underline fierce sense of urgency,” Sanders tells the crowd, “we will see in coming years more floods, more drought, more devastating wildfires, more famine, more rising sea levels, more ocean acidification, more extreme weather disturbances, more disease, more human suffering.”

Not exactly a cheerful message. But this we-must-do-better attitude suffuses Sanders’ entire political philosophy. He supports Medicare for All, because you shouldn’t have to refuse an ambulance if you can’t afford it. You “have a right to be angry,” because you’re living paycheck to paycheck while the richest Americans see their net worth skyrocket. There is no time. Something must be done now. This is unfair. This is wrong. These are refrains for all of the issues Sanders focuses on, validating the dissatisfaction so many feel.

Which is part of the reason why a self-described democratic socialist emerged in 2016 to give Hillary Clinton a fight for the Democratic nomination, why he’s tugged the party left in the years since and why he has a strong shot to win Iowa’s first-in-the-nation caucuses. At times this anger can devolve into something ugly. After this week’s debate, for example, when Sanders and Senator Elizabeth Warren argued over whether he’d said a woman couldn’t win the presidency, his surrogates took shots at Warren.

But to many supporters, Sanders is the only candidate who channels their pain. “People are hurting, and [Sanders] was able to speak to that hurt in a way that was authentic,” Varshini Prakash, the executive director of Sunrise Movement, which endorsed Sanders, told me after the Iowa City rally. “It feels often like we are just shouting into the void and politicians aren’t hearing us. … I think Bernie Sanders in 2016 was one of the first moments when my generation and people like me recognized that we could have a politician in office who cared deeply about what we cared about and wanted to fight our fight with us.”

The way Sanders has mirrored this anger differentiates him from his rivals. Yes, other Democrats are mad about all the same indignities. But where other candidates are promising love and unity, incremental steps toward their policy goals, and a return to normalcy after the Trump era, Sanders is promising to tear the system down. For his supporters, this is a virtue. For his critics, this is untenable. Whatever you make of it, this is Sanders, and he’s been like this all along.

“I think if you’re paying attention to what’s happening right now and you’re not angry, there’s something wrong,” says Nicole Khvalabov, 22, a University of Iowa student who attended the Iowa City rally. “I think a lot of other candidates spend a lot more time kind of sugarcoating what’s going on.”

As the 2020 campaign builds, this outrage at the injustice of it all will continue to be at the core of Sanders’ campaign. “There’s a tremendous amount of frustration, and he gives voice to that frustration because he authentically presents the reality that they’re facing,” Jeff Weaver, a senior adviser to Sanders, told TIME.

Just weeks out from when voters will start making their preferences known at the Iowa caucuses, Sanders is in a virtual four-way tie for first in the state, according to a recent poll by The Des Moines Register. His path to victory runs through communities that most often get marginalized — young voters, minority voters, low-income voters — and are therefore receptive to Sanders’ pitch. Polls show he’s performing well well with Latino voters. A recent FiveThirtyEight/Ipsos poll showed that more potential Sanders backers were low-income than for any other candidate in this week’s debate. The Register poll shows him leading with voters under 35.

Whether the “righteous anger” fueling Sanders’ movement will propel him to the nomination will become clearer in the coming weeks and months.

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Write to Lissandra Villa at lissandra.villa@time.com.