Standing in a reconstructed frontier fort on a cold and clear Tuesday morning, Bernie Sanders is promising a revolution. He'll bring in universal health care, make university tuition free and slam the hammer of the federal government down on Wall Street.

"We've got a handful of financial institutions that have so much economic and political power," the senator from Vermont and candidate for the Democratic presidential nomination declares, his Brooklyn-accented baritone echoing off the rafters to the rapt audience below. "We've got to break them up."

With less than two weeks to go before the Feb. 1 Iowa caucuses – the first electoral test of the U.S. presidential election – Mr. Sanders has good reason to be fired up.

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Once dismissed as a flash-in-the-pan long shot, the messy-haired 74-year-old has run a surprisingly durable campaign and has a real shot at embarrassing Hillary Clinton, the Democratic front-runner. One poll by the Des Moines Register last week showed the pair in a statistical tie in Iowa.

Losing the first contest of the race to Mr. Sanders would be a bad omen for a candidate as strong as Ms. Clinton, as well as a window into the divisions within the party.

The reasons for Mr. Sanders's rise, as explained by the Iowans who crowded into the Fort Museum on the edge of this farming and industry town of 25,000, range from his ambitious policy promises to his populist admonitions of capitalist greed to antipathy toward Ms. Clinton. The most-cited factor is the authenticity of the candidate himself.

Rusty Gregerson, a 45-year-old recycling-plant worker, points to the senator's penchant for riding public transit and flying coach as central to his appeal. The father of a 17-year-old daughter who's starting university next year, Mr. Gregerson also appreciates Mr. Sanders's promise of free tuition.

"It's just cool to see someone in that position who cares about us," Mr. Gregerson says. "Everything he says is genuine."

Key to Mr. Sanders's strength has been his popularity with younger voters.

Mia Dorothy, a 24-year-old insurance saleswoman, says she has never before gone to a political rally but was attracted by Mr. Sanders's health-care proposals. "I think he's got a real shot. For the first time in my life, it's a candidate I actually believe in," she says.

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To Mr. Sanders's supporters, Ms. Clinton is emblematic of the political establishment – happy to take donations from big banks and prone to compromising on progressive ideals.

"Bernie's authentic – he's stood on the same issues for 40 years – and Hillary seems to jump back and forth," says Samantha McMahon, a 25-year-old restaurant manager, who professed bafflement that Ms. Clinton, despite her own attempts at launching universal health care in the 1990s, opposed Mr. Sanders's promise in Sunday's Democratic debate to create a publicly funded health-insurance system.

"It was kind of funny that Hillary Clinton, who has been fighting for health care, didn't agree with Bernie," Ms. McMahon says.

Mr. Sanders's critics in the Democratic Party reply that his ideas, while they might be popular with the base, are simply not achievable in a gridlocked Congress and amid political polarization in Washington.

Ms. Clinton hinted at this on Sunday, when she described government-funded health care as a "contentious debate" she did not want to reopen.

At the Fort Dodge town hall, retired teacher Jane Nettleton, 72, points out to Mr. Sanders that he sat on the Senate committee that helped craft Obamacare and ultimately decided that full-fledged universal health-care would be impossible to get through.

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"Single-payer was not going to survive, so you went with the Affordable Care Act," she says. "How do you get these things passed?"

Mr. Sanders replies that his supporters have to keep the pressure on Congressional Republicans to change their minds. "This is what I mean when I talk about political revolution," he says. "The only way we get real change in America is when millions of people stand up and demand it."

Afterward, Ms. Nettleton says she thinks Ms. Clinton's practical incrementalism is a better bet: "We need more of a pragmatist, not an idealist," she says.

Mr. Sanders's campaign may very well be a quixotic quest. Despite his strong numbers in Iowa, he is still far behind nationally; a Monmouth University poll this week pegged Ms. Clinton at 52-per-cent support among Democratic voters, compared to 37 per cent for Mr. Sanders.

Dennis Goldford, a political science professor at Drake University in Des Moines, attributes Mr. Sanders's relative strength in Iowa to the state's overwhelmingly white demographic: Mr. Sanders's focus on economic issues has resonated with white working-class voters, but he has been weak on race and gender, while Ms. Clinton commands more support with African-Americans.

"He's an unreconstructed New Dealer, who just wants a stronger social-safety net than the Democratic Party has [advocated] over the last 25 years. He appeals to the Democratic id," Prof. Goldford says. "He's a bit of a one-note samba. He doesn't talk about identity politics or gender politics as comfortably as Hillary Clinton does."

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Still, a Sanders victory in Iowa, even if Ms. Clinton ultimately wins the nomination, would be "a warning sign" to her that she has to do more to speak to Americans' economic concerns and appeal to white working-class voters, Prof. Goldford says.

For his part, Mr. Sanders is eager to remind his audience that, a year ago, no one even thought he would get this far.

"When I began the campaign, they said … 'Secretary Clinton is the inevitable candidate of the Democratic Party. You might be able to raise some good issues, but you can't win,'" he says. "Well, today the inevitable candidate does not look so inevitable."