DES MOINES, Iowa — The greatest trick Bernie Sanders ever played was convincing the world the chance he could win didn't exist.

The independent senator from Vermont has been running for president for the better part of five years, but some Democrats are only just now, a week out from the first contest in the 2020 presidential primary season, beginning to come to grips with the fact that he could actually win the nomination.

"Suddenly, we have the Democratic establishment very nervous about this campaign. We got Wall Street nervous," Sanders told a crowd of roughly 1,100 Sunday night in Sioux City. "They're starting to think, could this really happen?"

"We are their worst nightmare," he added.

Next Monday's caucuses remain a toss-up, according to the polls. But Sanders has taken the lead in several recent surveys of Iowa and New Hampshire and continues to be the field's best fundraiser — without having faced the same intensity of incoming fire as some other candidates, like fellow progressive Elizabeth Warren.

A Sanders win would turn the Democratic Party upside down, much as Donald Trump's victory did for the GOP in 2016. But how could virtually no one see Sanders coming when he nearly overturned the party establishment four years ago when he came within a whisker of winning Iowa?

"They've always wanted to discount or dismiss him, but they do so at their own peril," Hugh Espey, executive director of Iowa Citizens for Community Improvement, which first brought Sanders to Iowa in 2014. "It's staring them right in the race."

For most of the 2020 primary, everyone from Wall Street to K Street viewed Warren, a senator from Massachusetts, as the more formidable contender, leading to a storm of scrutiny when she was surging in the polls last year.

Given Sanders' lackluster poll numbers then, moderate Democrats and Republicans often built him up to use as a cudgel against Warren, especially when she was struggling to explain how she'd pay for "Medicare for All."

"At least Bernie's being honest here," Sen. Amy Klobuchar of Minnesota said at the Democratic primary debate in October, while joining others on stage in criticizing Warren.

Billionaires like former Goldman Sachs Chief Executive Lloyd Blankfein and investor Leon Cooperman also trained their fire primarily on Warren, not Sanders, presumably assuming, like so many others, that Sanders' time had passed and that he would soon hand off the baton of the progressive movement he started to Warren.

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"None of his opponents have attacked him," said Ian Sams, a Democratic operative who has now gone two rounds against Sanders — as a campaign aide to Clinton in the last election, and to Sen. Kamala Harris, D-Calif., in this one. "There had been a supposition all last year from media and the political class that there's no way Bernie way could win, which was a pretty faulty assumption."

The attacks worked to weaken Warren, but left Sanders relatively unscathed.

Now, some moderate Democrats feel the need to sound the alarm and try to wake supporters up to the fact that Sanders is not a mere protest candidate, but a real threat to win the nomination and, they argue, potentially cost Democrats the election against Trump.

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"Bernie Sanders could be the nominee of our party," Pete Buttigieg's campaign alerted supporters in an email. "Bernie's campaign is out-raising and out-spending us. If this continues, there's a good chance he wins the Iowa caucuses."

The tone of the warning, which was repeated in a second email and follow-up text message to supporters, was jarring in a campaign where candidates have rarely gone after each other by name.

Asked by NBC News if he was worried that Democrats were underestimating Sanders' appeal, Buttigieg said, "It's a terrible mistake to not take any candidate seriously."

Klobuchar suggested that nominating Sanders could hurt down-ballot Democrats. "I do not come from a state that is as blue as Vermont," she told reporters in Ames. "I have been able to get those votes and bring them in. And so I think a lot of people have talking points about how they can do this. I actually have the receipts."

Matt Bennett, the vice president of the centrist Democratic think tank Third Way, which has agitated against Sanders and his philosophy for years, said many political insiders have a Sanders-size "blind spot."

"We issued a warning a year ago that Sanders could win the nomination and would likely lose to Trump. And we've been the only ones really taking the fight to him," Bennett said.

"It's past time for other Democrats to come off the sidelines and for the media to start doing its job to vet a serious contender for the nomination," he added. "We simply can't stand by while there's a threat that Democrats could nominate a guy who would hand such nuclear-level ammunition to the Trump campaign."

Voters at events for moderate candidates this weekend expressed concern about Sanders' potential nomination, though all said they would vote for him in the general election.

"It's going to be hard for him to pull in people in the middle," Todd Darson, who is deciding between Biden and Buttigieg, said of Sanders. "That's what scares me the most.'

Anything could happen in the Feb. 3 caucuses, and Iowa Democratic insiders say it's just as likely that former Vice President Joe Biden could win the caucuses and then quickly consolidate the nomination as it is likely that Sanders could win Iowa.

But Sanders, on a whirlwind tour of Iowa this weekend during a break from his jury duty in the Senate's impeachment trial, relished the fear that he said he has been striking in his enemies.

"They're looking at recent polls in New Hampshire and in Iowa, and they're saying, 'Oh my God, Sanders can win!'" the senator told a packed auditorium in Ames.

Sanders' allies say the political and media elites miss his strength because they have a blind spot for candidates whose message is aimed at working-class voters, including Sanders, but also Biden, whose durability has surprised many, and Trump.

"The establishment has underestimated him because the Beltway fails to grasp how much support there is for the principle that every American should have basic health care, basic education and the opportunity for a good paying job," Rep. Ro Khanna, D-Calif., a co-chair of Sanders' campaign, told NBC News.

So far, a "Stop Sanders" effort has not emerged, but one still could — and Sanders allies say they now have a target on their backs.

"Things are going to get crazy," said Rep. Alexandria Ocasio-Cortez, D-N.Y., who has endorsed Sanders and become one of his top surrogates, told volunteers at a field office opening in Ankeny, Iowa.

Michael Moore, the documentary filmmaker, who joined Sanders as a warm-up act on his most recent Iowa swing, told supporters in Ames that "the rich" are going to start panicking.

"The knives are out," Moore said.