BURLINGTON, Iowa — The candidates coaxing Iowans to caucus for them on Monday rely on historical touchstones to make their case.

Sen. Bernie Sanders (I-Vt.) hails the ambition of the New Deal. Joe Biden invokes the triumphs of the civil rights movement in speaking of the “soul of the nation.”

Andrew Yang harks back to the Spanish flu. The 1918 pandemic, he says, is the last time American life expectancy declined three years in a row, as it did in the years after 2014. (It rose again in 2018, the Centers for Disease Control and Prevention reported Thursday.)

“It ordinarily goes up because you’re getting stronger, richer, healthier,” Yang told about 40 people in this small city on the banks of the Mississippi River where the disappearance of industry has left deep scars. “But here in the U.S., it’s gone down and down and down.”

The parallel captures the doomsday warning at the crux of his campaign, which has transformed the entrepreneur and former tech executive from a political unknown into an Internet sensation and fundraising phenom known for his “Freedom Dividend,” his signature plan to give every American $1,000 a month. Already he has outlasted three sitting U.S. senators — a fact he points out on the stump, sometimes naming the fallen, as he treks across Iowa on a 17-day bus tour leading up to the first-in-the-nation caucuses.

Yang’s dark vision is at the heart of his success. His staying power, despite surveys showing him to be a long shot, shows how his campaign has captured the febrile state of the Democratic electorate. Voters flocking to the political novice are petrified not simply by the prospect of President Trump’s reelection but by the feeling that the economic and social dislocation that powered his rise are beyond their control, perhaps even beyond the control of Washington — which Yang wants to break up by redistributing federal agencies throughout the country.

They are right to be afraid, according to Yang, whose carefree demeanor (he crowd-surfed in California last fall) and online antics (“It hasn’t dawned on me that over a million people follow me on Twitter,” he said in an interview) mask the severity of his message.

“People are not excited about the future that lies ahead,” he said in Burlington, where he draped himself in a scarf imprinted with the American flag and marveled that it took “the futuristic Asian man” — as he put it — to deliver a harsh verdict on the country’s direction.

“Homelessness, stress, anxiety, mental illness, depression, suicides, drug overdoses,” said the 45-year-old father of two, enumerating the consequences of what he calls the “fourth industrial revolution,” borrowing a phrase introduced by economists and engineers to describe breakthroughs in robotics, artificial intelligence and advanced communication technologies.

Yang, who speaks the language of Silicon Valley and recounts conversations with SpaceX founder Elon Musk, is no Luddite. But he can also channel a populist antipathy to major technology companies, while tacking on a psychological diagnosis, arguing that the machines meant to liberate us have instead made us miserable.

That sense of impending doom is profound in Burlington, where John Berry, an MRI tech, said he had recently come across something online that terrified him — a video showing a shipyard in China that was fully automated. “There were no people necessary,” said Berry, 49, who plans to caucus for Yang.

The existential anxiety that binds the candidate’s devoted followers, known as the “Yang Gang,” also points to the direction his supporters may go in precincts where they do not reach the required 15 percent to gain delegates. In a tight race, with four candidates bunched near the top, campaigns polling in the single digits — as Yang’s has — could end up being kingmakers if their supporters all rally around the same second-choice pick.

Especially among young people, who are some of his most enthusiastic supporters, many see Sanders as their next best hope because of his anti-establishment bona fides. Yang gestured at the shared appeal, saying this week, “I think that Bernie and I do have a lot of overlap in support, so it wouldn’t be surprising to me if many of our supporters head in that direction.”

Sam O’Connor, a junior at the University of Iowa, had come to see Yang at an appearance this week on campus. “The only other option would be Bernie,” he said.

One floor below, an organizer for the Sanders campaign was signing up students to volunteer.

While Yang cautions so urgently about the future, his campaign also seems to inhabit it more fully than any other. Eric Ming, the campaign’s digital director, credits “online community building” for the campaign’s early strength. “I’m not going to pretend like we have any special sauce,” he said, describing how the sea of blue baseball-cap emoji dotting pro-Yang Twitter, along with other online branding efforts, are useful in “communicating with supporters about what’s going on in the race and how they can help.”

O’Connor, an English and creative-writing major, said he first heard about Yang on Reddit. Gabrielle Aranza, a sophomore studying marketing and theater arts, came across Yang on YouTube, where she clicked on a video of his conversation with Ben Shapiro, the conservative commentator.

Aranza’s second choice would also be Sanders, who has a formidable online following of his own.

 “I have a lot of respect for Bernie,” said the 19-year-old. She doubted that all of Yang’s supporters would be willing to switch their allegiances, though, even in precincts where they don’t muster enough support for what’s called “viability” and are allowed to choose another candidate in a realignment process. “A lot of people I’ve met only care about Andrew,” she said.

Yang’s best chance of winning delegates is in urban centers such as Iowa City, said Ed Cranston, the Democratic chair of Johnson County, which includes the University of Iowa. But student support isn’t enough to sustain him in many precincts, Cranston said.

“As far as being viable, it’s going to be a tough one because of that 15 percent,” he said. “I would imagine he could hit 15 percent in some precincts, but I wouldn’t think it would be many.”

Cranston said that along with the 78-year-old Sanders, Pete Buttigieg appeals to some of Yang’s followers drawn to a younger candidate able to speak to the concerns of different generations. Nick Pryor, the president of the University of Iowa College Democrats, said he also sensed some interest in Sen. Amy Klobuchar (D-Minn.), suggesting the “Yang Gang” may not be as unified as it seems online.

In Burlington, it’s the mission of Desiree Stumpf, a precinct captain for Yang, to make sure his supporters don’t have to find a second choice Monday. The 44-year-old head of nursing at the local hospital has a list of 45 committed caucus-goers in her precinct. She’s hunting for more.

“He’s not there yet,” she said. “But it’s a season of political surprises. I’m hoping it’s a surprise for him.”