With recent polls showing him taking leads in early states like Iowa and New Hampshire, as well as delegate-rich states like California, Vermont Sen. Bernie Sanders is increasingly looking like a major threat to win the Democratic primary. This is a well-timed burst of enthusiasm for the candidate, seeing as the Iowa caucuses are less than a week away. A win in Iowa could provide Sanders with the momentum to overcome Joe Biden’s so-called firewall in southern states like South Carolina, where the former vice president’s popularity among Black voters continues to give him a commanding lead.

For much of the race, Sanders trailed Biden and Massachusetts Sen. Elizabeth Warren, only recently surpassing the latter to become the clear leader in the progressive lane of the primary. He has faced very little in the way of direct attacks, both because he hasn’t been the frontrunner and because other candidates are wary of alienating his passionate supporters, who could be crucial to their own chances down the line. This marks a substantial difference from 2016, when he faced continued attacks from Hillary Clinton over his record on guns and other issues, as the two were the only major candidates in the Democratic field. Meanwhile, Warren has taken far more heat than him on Medicare-for-All — which broke into the mainstream as one of Sanders’s core positions.

Maybe this all congealed into a sense of complacency among his opponents. Why attack the guy who’s not winning, not surging, and has lost before? Now, however, the establishment-oriented moderate factions of the Democratic Party are finally scrambling to mount a defense against Sanders as he climbs in the polls. Sanders is far more progressive than any Democratic nominee in modern history, and his popularity has been a shock to the party establishment. That shock is beginning to turn into action.

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According to a report in The Daily Beast on Monday, a group of Democrats have begun floating the idea of a coordinated attack on Sanders if he wins the Iowa and New Hampshire primaries. The effort involves an assortment of establishment-oriented Democratic operatives and mover-and-shaker types concerned that Sanders could be a weak candidate in the general election. “It’s not any one group of people,” an anonymous source told the Beast. “It is a loose network of people who think he would be very problematic as a nominee. There is, at this stage, calls to, and conversations with, donors and organizations that might support such an effort. But as far as I can tell, nobody has found sufficient financial support to get it off the ground.”

But on Tuesday, one such group launched a new anti-Sanders ad campaign in Iowa. A pro-Israel group called Democratic Majority for Israel will spend almost $700,000 on an ad that shows Iowa residents describing their worries about Sanders’s candidacy. One person mentions his 2019 heart attack as a concern.

Mark Mellman, the president of Democratic Majority for Israel, told Politico that the point of the ad is to hear directly from voters regarding their fears that Sanders might lose to President Trump in November. “The ad raises serious questions about his electability in their own words. Health is one of the things that people raised. But in general people say they like Bernie Sanders, they respect him but they say he can’t win,” Mellman said.

Ads like this one may be too little, too late to have an effect on Sanders’s surge in Iowa, though. The first in the nation caucus will happen Monday. All in all, the senator has faced almost no concerted opposition and seems well placed to win at least two of the first four primaries.

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“I think the Occam’s razor solution to the mystery of why Dem elites aren’t acting strategically against Bernie is that Dem elites aren’t as anti-Bernie as these articles suggest,” tweeted Vox co-founder Ezra Klein. “A few are, but not enough to make some kind of blocking action possible, much less effective. In general, I think this conversation is backwards: The question isn’t why Dem elites haven’t united against Bernie, but why they haven’t united for Biden. This primary is being shaped as much by Biden’s weaknesses as anything else.”

Klein is right that Biden’s inability to emerge as the consensus establishment choice — as Clinton did early in 2016 — has paved the way for Sanders’s late-breaking surge. If the senator is able to snatch a few early victories, he could quickly knock some of his top rivals — like Warren or Pete Buttigieg, the former mayor of South Bend, Indiana — out of the race. This could set up an interesting dynamic in which the senator goes head to head with Biden and possibly Michael Bloomberg, the former New York City mayor who has the cash to stay in the race as long as he likes, and whose close friend Steve Rattner recently claimed he is only staying in the race to take votes from Sanders.

Sanders’s surge has led to a recent outpouring from a chorus of mainstream liberal media voices warning that he’s weak in the general election. On Monday, an article titled “Bernie Can’t Win” emerged at The Atlantic, written by David Frum, a former speechwriter for former President George W. Bush. On Tuesday, in a piece calmly titled “Running Bernie Sanders Against Trump Would Be an Act of Insanity,” New York’s Jonathan Chait argued that “the totality of the evidence suggests Sanders is an extremely, perhaps uniquely, risky nominee,” pointing to the fact that Sanders has not previously run in a general election against an opponent who will attack him harshly over his support for democratic socialism. Echoing them both was Will Saletan in Slate, arguing Tuesday that “Sanders has major liabilities that haven’t been exploited in the primaries. If he’s the nominee, those liabilities could hand the election to President Donald Trump.” All of these writers hail from somewhere on the center-left side of the spectrum, and all of them claim to be divining the will of centrist swing voters with similar views.

All of this is leading towards a fascinating whirlwind in which a perpetual underdog candidate appears to be on the verge of some seriously crucial wins, potentially spurring a backlash from his own party.

Even Clinton seemed to join this rising chorus. News emerged of comments she makes in her upcoming Hulu documentary about her erstwhile presidential rival; she apparently says of Sanders, “Nobody likes him, nobody wants to work with him, he got nothing done. He was a career politician. It’s all just baloney, and I feel so bad that people got sucked into it.” Granted, the actual interview for this documentary took place over a year ago, and Clinton has promised to stand by Sanders if he’s the Democratic nominee. But her bitter feelings toward him — especially the phrase “nobody likes him,” which clearly isn’t referring to voters but rather to the amorphous group of high-profile politicians and D.C. figures to which she belongs — capture a similar sense of frustration with Sanders’s far-left ideology from those in the Democratic mainstream.

All of the anti-Sanders articles were written by people with major ideological differences from Sanders, though they positioned themselves more as dispassionate observers of the state of play. Clinton, obviously, has a personal history with the candidate. This has led Sanders’s supporters to argue that these figures are trying to disguise ideology within horse-race analysis.

“We see a spasm of frantic and desperate attacks on Bernie Sanders,” tweeted Sanders’s speechwriter David Sirota. “These attacks seem disparate, but what ties the various criticisms together is that they are all sponsored/amplified by zillionaires and corporations that want nothing to fundamentally change.”

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Meanwhile, media reports have painted Sanders’s followers as aggressive cyberwarriors, including a recent New York Times feature arguing that “his colossal online support base has been by turns a source of peerless strength and perpetual aggravation.” Supporters in turn have often felt unfairly targeted by the media, and have often referred to the idea that Sanders has been hit with a so-called “Bernie Blackout” of coverage; his actions, they claim, are always couched in a negative way, while those of his opponents are given a positive light.

All of this is leading towards a fascinating whirlwind in which a perpetual underdog candidate appears to be on the verge of some seriously crucial wins, potentially spurring a backlash from his own side of the aisle. Despite the fact that he made a serious run for the presidency in 2016, and is in the midst of another strong campaign this time around, a huge swath of political tastemakers haven’t really significantly reckoned with the prospect of a President Sanders.

It’s led to an escalating heat between Sanders’s supporters and their opponents online. If the attacks that have emerged against him this week are any indication, the road for him is only going to become tougher — and the rhetoric possibly more brutal.