DES MOINES, Iowa — Bernie Sanders supporters are warning that a lack of enthusiasm for Joe Biden could bring about the same result that nominating another establishment Democrat did four years ago: electing Donald Trump.

Sanders backers fret the socialist Vermont senator, 78, is being consistently ignored and underestimated by Beltway insiders and a Washington-based press corps — the same bunch, in their view, who failed to predict Trump's 2016 White House win and were sure Hillary Clinton, who beat Sanders for the Democratic nomination, would easily prevail.

They're particularly steamed Sanders, a socialist who has been in Congress since 1991, isn't getting his due though he's proven to have staying power in public opinion and fundraising, even after suffering a heart attack last October . As of last week, he leads a tight four-horse race in Iowa ahead of the first-in-the-nation caucuses on Feb. 3, raising a record figure of $34.5 million in the final financial quarter of 2019 for a total of $96 million.

Nominating a career center-left candidate such as Biden, the former vice president and 36-year Delaware senator, would dampen enthusiasm by the liberal populist grassroots, Sanders backers contend.

Joe Jackelen, 35, backed Sanders in the 2016 primary and wrote him in on the ballot that fall because he didn’t like what Clinton “stood for.” He said he only regretted that decision “to a degree,” despite it helping give rise to Trump, who beat Clinton in Iowa 51% to 42%.

Jackelen, an IT professional from Ames, said Biden’s refusal to call billionaires “the enemy” was one reason his potential nomination in 2020 could suppress the left-wing vote.

“When I look at some of these other candidates out there, I can see them just sucking any enthusiasm from any undecided voters. So anyone who might have been for Bernie won’t vote for Biden, or any of the others out there,” Jackelen told the Washington Examiner after a Sanders town hall in Newton.

“I think that people will sit at home because they see establishment Democrats that don’t really care. The only thing that they bring to the table is that they’re not Trump. That is not going to drive enough people out that will get a win over Trump,” he said.

Kay Pence, 65, said she's unconcerned about the "Red Scare" stigma attached to socialists such as Sanders. Yet, she believed many Iowans were still “discouraged” about politics because they “didn’t see a lot of hope and change” after President Barack Obama’s inauguration in 2009, she said.

“A lot of people are backing away from voting,” the retiree from Eldridge said after a Sanders rally in Davenport.

The recalcitrance of Sanders supporters coincides with his team’s decision to break a nonaggression pact with ideological ally , Massachusetts Sen. Elizabeth Warren, crafting campaign strategy around understood holes in her candidacy. The pair then clashed over Warren’s claims Sanders questioned whether a woman could win the White House. Sanders denies her allegations . Her confrontation with him during and after this week’s Iowa debate caused pro-Sanders and unaffiliated Twitter trolls and bots to push the #NeverWarren and #WarrenIsASnake hashtags to No. 1 in the United States.

For Jon Reinish, managing director of political consulting firm SKDKnickerbocker, “the extreme devotion combined with extreme toxicity of his diehard supporters” had perpetuated the perception that Sanders and his fans “kneecapped the Democrat in the general election,” particularly in Michigan, Pennsylvania, and Wisconsin. History seemed to be repeating itself, he said.

Reinish, however, warned about the role social media was having in “sowing this division among Democrats” and “exacerbating these wedges.”

“Yes, that ‘Bernie or bust’ movement does exist out there, but there needs to be far greater scrutiny into how much of this is actually real and how much is absolutely being played into,” he continued.

The Democratic National Committee started promoting post-primary unity last November, one year out from Election Day, when the field of hopefuls converged on Des Moines for the Liberty and Justice Celebration, an Iowa Democratic Party fundraiser. At the time, Sanders donated to the state party but hosted a competing watch party close to the main event.

Meanwhile, with the vast majority of Democrats preoccupied with the issue of electability, likely caucusgoers at a Pete Buttigieg town hall in Des Moines said they hoped the party would come together to defeat Trump.

“Bernie’s had his chance four years ago. He’s 78; he had a heart attack. Come on. His electability is not there. His demographic is the 20-something age group, right? Who are the ones who get out and vote or go caucus?” Bryan Crowder, 51, asked the Washington Examiner.

Crowder, a Des Moines insurance analyst, chalked up the chatter as a tactic “to make Bernie more appealing.”

“‘If Bernie doesn’t get the nomination, well, I’m not going to vote.’ How did that work for you last time? We’ve got to stop letting 70-year-old white men rule the presidency, and the Senate, and the House. It’s time to usher in that new era we’ve been talking about,” he said.