DES MOINES, Iowa — As Iowans prepare to caucus on Monday, Democrats are facing a Bernie Sanders problem.

The 78-year-old Vermont socialist, who many in the party view as a risky bet in the general election against President Trump, could win both Iowa and New Hampshire. He’s been leading in recent Iowa polls, is drawing the largest and most passionate crowds, and has a deep organization that he’s been building since he nearly won the state four years ago. Should he win here, he’ll be the heavy favorite to take New Hampshire, which he won easily in 2016. Since the first caucuses in 1972, no Democrat who has won the first two states has ever lost the Democratic nomination.

However, even if party elites get their wish and Sanders is stopped, his supporters are unlikely to take any defeat lying down. In the days running up to Iowa, bitter divisions from the 2016 presidential election have resurfaced, and Sanders supporters and surrogates have publicly lashed out at Hillary Clinton and the Democratic National Committee.

This is something of a nightmare scenario for Democrats.

After facing accusations of having rigged the 2016 primary in Clinton’s favor, the DNC instituted several changes to avoid the appearance of tipping the scales in 2020. The hope among many Democrats was that Sanders would fizzle out naturally and that the rules changes would mollify his supporters, who would then be more willing to turn out for the eventual nominee. In 2016, some die-hard Sanders supporters stayed home in an election in which the decisive states went to Trump by razor-thin margins.

Thus, Democrats may be caught in the box they were hoping to avoid: Either Sanders wins the nomination, and they have to face Trump with an avowed socialist at the top of their ticket, or Sanders gets denied the nomination and his embittered supporters contemplate bailing on the eventual nominee in November.

To be sure, it isn’t necessarily true that Sanders would be the weakest general election candidate. In making the case as to why he would be the strongest candidate to defeat Trump, Sanders said at an Indianola campaign stop on Saturday, “Our campaign is the campaign of energy, is the campaign of excitement, is the campaign that can bring millions of people into the political process who normally do not vote."

His case is not totally without merit. In 2016, Trump was able to turn out voters who weren’t accounted for by experts. Sanders has a similar anti-establishment populist appeal and is the only Democrat who generates the sort of passion from his supporters that one sees at Trump rallies. As Sanders surrogate Democratic Washington Rep. Pramila Jayapal has been putting it at campaign stops, “We have to move past the myth of the likely voter to the truth of every voter, and that means expanding our electorate and bringing more people in.”

Yet, while Sanders is undoubtedly the only Democratic candidate who is able to attract a level of passion among his supporters that rivals Trump, it also would be a huge risk to bet that Americans are ready to put a socialist in the White House.

Among other things, take healthcare. Democratic gains in the House in 2018 were fueled by the backlash against Trump among suburban voters. Candidates in swing districts rejected the idea of “Medicare for all” and focused more narrowly on promising to protect people with pre-existing conditions. Yet, a Sanders-style healthcare plan is like a heat-seeking missile directed at the very suburban voters Democrats are hoping to win, who overwhelmingly have good insurance through their employers that would be eliminated.

In December, I looked at census data and found that in eight politically significant counties in the swing states of Michigan, Wisconsin, and Pennsylvania with a high concentration of suburbs, the uninsured rate was half the national number — or 5.2%. And 81.3% of those with insurance had private coverage. Suburban voters may hate Trump, but do they hate him enough to support somebody who will raise their taxes and strip away their current insurance plans that they generally like?

Brian Kading, an Iowa resident who is leaning toward supporting Pete Buttigieg, channeled what many Democrats were thinking about Sanders, when he told me in Indianola, “Sometimes I think he's the only one that can win, because I think he can generate a lot of enthusiasm. And then other times, I think he can be easily painted as too extreme on socialism and will not only lose the presidency, but lose the House and the Senate, and probably even further down the ticket, just, it could be a major disaster for the party.”

So that’s the big risk in nominating Sanders. If this new wave of voters he’s promising does not actually show up in November, the results could be catastrophic.

Yet, should establishment Democrats work to deny Sanders the nomination to stave off this reality, they could be turning away his supporters. As things stand, one recent poll found that just 53% of Sanders supporters said they definitely would back the Democratic nominee were it somebody else. In contrast, 87% of Joe Biden supporters and 90% of Massachusetts Sen. Elizabeth Warren supporters have said they would support whoever Democrats nominate.

Courtney Langel, who was at a Sanders event on Friday night in the Des Moines suburb of Clive, said in 2016 she caucused for Sanders but didn’t vote for Clinton in the general election. She plans to caucus for him again and said if he isn’t the nominee, she would have to “do more research” before deciding whether to vote for the Democratic nominee.

Jason and Marci Willey not only caucused for Sanders in 2016 but wrote him in in the general election rather than vote for Clinton. When asked before a Saturday night event in Cedar Rapids whether they would support the eventual nominee this time around if it isn’t Sanders, Jason said, “We’ll have to see how it goes.” Marci added, “If the establishment kind of cheats again, you never know.”

This is the problem for Democrats. Any steps that party elites take to undermine Sanders will only inflame his supporters, who will attribute any loss to him being robbed by the corporate wing of the party. That was on full display at a Friday night Sanders event in Clive, which followed a Politico report that some DNC members had begun discussing a rules change aimed at weakening Sanders.

In one of the speeches, filmmaker Michael Moore ripped the DNC for changing the debate rules in a way that could allow Michael Bloomberg to participate, “Because he has a billion f---ing dollars.” At the mere mention of the DNC, Sanders supporters in the crowd started booing. Moore then went on an extended rant, to a cheering audience of thousands, about how it was bad enough trying to fight Trump, Republicans, Wall Street, and corporations, but now they also had to fight, “the corporate Democrats — the 1% of the Democratic Party who are thoroughly pissed that Bernie Sanders is now No. 1.”

Later, at the same event, Democratic Michigan Rep. Rashida Tlaib joined the crowd in booing Clinton from the stage after being asked about the 2016 Democratic nominee’s recent comments that nobody liked Sanders. That episode triggered a new round of back and forth between Sanders and Clinton camps that spilled into the weekend.

What’s especially dangerous for Democrats is that their system that allocates delegates on a proportional basis by state and congressional district makes it more likely that no candidate will get a majority of delegates before the convention. And if Sanders ends up being denied the nomination on the convention floor, the backlash could make 2016 look like a joke.

Tim Carney contributed to this report.