Days before Democrats begin voting for a nominee to take on Donald Trump, panic is beginning to spread in some corners of the party that their standard-bearer could be a 78-year-old democratic socialist from Vermont.

As Bernie Sanders shows signs of strength ahead of Monday’s Iowa caucuses, anxious Democrats fear a repeat of 2016, when establishment Republicans failed to grasp Trump’s strength before it was too late.

They argue that Sanders would be a “uniquely” flawed and woefully “untested” nominee and easily crushed by what will certainly be a well-financed negative assault.

“There is a deep treasure trove of stuff from Bernie’s background – all of the radical things he’s said and done over the years – that the Trump campaign could circulate,” said Matt Bennett, the executive vice-president for public affairs at Third Way, a center-left thinktank. “He is without question the candidate Trump hopes to have as an opponent in November.”

Bennett is among an increasingly vocal group of Democrats who believe nominating Sanders is not only a risk but a potentially “historic mistake”.

In a memo sent to influential Iowa Democrats, Third Way warned that Sanders’ “politically toxic background” and “far-left positions” would repel moderate and independent voters who are critical to Trump’s re-election.

That sentiment was amplified this week by a series of columns and editorials and by the Democratic Majority for Israel, a group of pro-Israel Democrats that is spending nearly $700,000 to air a television ad in Iowa ahead of the caucuses that raises doubts about Sanders’ electability – and physical fitness.

“I like Bernie, I think he has great ideas,” Michael Kuehner, an Iowa voter, says in the ad. “But Michigan, Pennsylvania, Iowa? They’re just not going to vote for a socialist.” Another voter, Darby Holroyd, says: “I do have some concerns about Bernie Sanders’ health, considering the fact that he did have a heart attack.”

Joe Biden has remained durable atop the Democratic field and maintains a clear path to the nomination. But recent surveys show Sanders leading in Iowa and far ahead in New Hampshire. On Friday, a NBC News/Wall Street Journal poll found Biden and Sanders in a dead heat at the top of the Democratic field.

The urgency of the centrists’ warnings against Sanders reflects a realization he could actually win the Democratic nomination.

“There’s a lot of history to suggest that if he wins those two states, he becomes very hard to stop,” said Mark Mellman, a veteran Democratic pollster and the president of Democratic Majority for Israel.

After a summer of slumping poll numbers and a heart attack in October, few expected Sanders to rebound. Establishment-minded Democrats instead trained their fire on his progressive rival, Elizabeth Warren, attacking her economic vision and hammering her on the details of her healthcare plan. The effect was to turn the candidate with the most loyal supporters and largest fundraising operation into an underdog – allowing him to run as an anti-establishment insurgent rather than a frontrunner.

“Suddenly, we have the Democratic establishment very nervous about this campaign,” Sanders roared at a rally in Sioux City, Iowa, last weekend. “They’re starting to think: could this really happen?

“We are their worst nightmare,” he said, to deafening applause.

The Democrats behind the recent moves to dim Sanders’ rise insist there is no coordinated “stop Sanders” effort, and few believe such a movement would work. Instead, these worried Democrats say their goal is to highlight Sanders’ vulnerabilities for a small but significant slice of undecided primary voters who are open to his message but not enmeshed in his “political revolution”.

However, few Democrats are joining their public call to arms, partly out of fear that any indication of an organized anti-Sanders campaign would antagonize his loyal army of supporters, who are already deeply suspicious of the party establishment. Others believe such efforts only serve to hand Sanders a political gift.

Sanders raised $1.3m after a fundraising appeal asking supporters to fight back against attacks by “outside groups”. On Thursday, a coalition of nine outside groups backing Sanders said they would join forces to build what they called the “largest independent mobilizing commitment” in 2020.

Mellman said he recognized the risks of inflaming Sanders supporters, but believes remaining silent “would be worse”.

“Democrats say defeating Trump is what’s most important to them,” he said. “We think Sanders is very poorly positioned to do that.”

At a town hall in Decorah, Iowa, on Thursday morning, Pete Buttigieg lamented that Sanders adheres to “a kind of politics that says you’ve got to go all the way here and nothing else counts”.

Later that day, at a campaign stop in Pella, Iowa, Biden was unusually blunt when asked to draw a contrast with Sanders: “I’m a Democrat.”

Sanders, an independent who caucuses with the Democrats, has signed a loyalty pledge to serve as a Democrat if elected president. But Biden pressed: “He’s not a registered Democrat, to the best of my knowledge.”

John Delaney, the former Maryland congressman and a long-shot presidential candidate, departed the field on Friday with a warning that Democrats embrace the economic populism of Sanders or Warren at their peril.

“Their proposal takes healthcare away from a lot of people and forces them on some new government plan,” he said of Sanders and Warren’s support for a government-run healthcare model. “That’s a hard way to win an election.”

Sanders could carve a similar path to the nomination as Trump in 2016, by taking advantage of a fractured field with no clear leader. But the 2018 congressional elections delivered mixed results for Sanders’ movement, with little evidence to support his confidence that a populist leftwing agenda will resonate in conservative battlegrounds.

“This isn’t an election where you win with huge turnouts in California or Massachusetts or New York,” said Harold Schaitberger, the general president of the International Association of Firefighters, a prominent union that is rallying support for Biden in Iowa. “People may not like it, but this is an electoral election. To win, you need a nominee that can compete in Pennsylvania, Ohio, Michigan, Wisconsin, Iowa.”

That analysis is sharply contested by Sanders supporters, who point out that running a centrist candidate failed to beat Trump in 2016, and that the eventual nominee must be someone who can energize young and disaffected voters to counter the president’s fervent base. Once again, they believe, the political class has underestimated Sanders’ appeal to working people.

“The political establishment has ignored working people for so long. They sowed these seeds,” said Hugh Espey, executive director of Iowa Citizens for Community Improvement, which has endorsed Sanders. “We’re sick and tired of business-as-usual politics. We need to do something different.”

Speaking at an event on behalf of Sanders on Wednesday night, the climate activist Naomi Klein cautioned that the attacks were just the beginning.

“At every turn, Washington DC elites said Bernie’s position was unrealistic, impossible, not credible,” she intoned. “But over and over again over the past 40 years history has sided with Bernie Sanders.

“If he was right about the past,” she concluded, “he is right about the future too.”