The Internet is not real life. Social media, in particular, is populated with political content by an unrepresentative sample of disproportionately energetic political maximalists, most of whom skew far to the left. When possible, it’s best practice not to look to cyberspace to spot emerging trends or support preexisting biases.

But sometimes, the Internet insists on its primacy and ignoring it doesn’t always make it go away. That has been Bernie Sanders’s unenviable experience over the past week as his prospects for securing the Democratic Party’s presidential nomination have strengthened.

On Monday morning, former New York City Mayor Mike Bloomberg’s presidential campaign launched an online ad targeting Bernie’s excitable online supporters. The spot features Sanders fans at their worst: calling for the creation of “lists” of Bloomberg’s campaign staffers in the effort to intimidate them out of working for his candidacy, crafting tombstones for Joe Biden, mocking Pete Buttigieg’s appearance, and hurling epithets at just about every Democrat who failed to genuflect at the altar of democratic socialism. It concludes with the implication that, despite Sanders’s appeals to civility, the toxic climate around his campaign did not evolve ex nihilo.

Let’s stipulate that Bloomberg is among the worst ambassadors for this message. If the former mayor’s candidacy is distinguished by anything beyond his willingness to spend exorbitant sums of his own money to avoid competing in the early states, where cash advantages can be overcome through sweat equity and a passionate base of support, it’s Bloomberg’s willingness to get down into the mud. He has responded to Donald Trump’s personal attacks in kind. He is dismissive of the intelligence of whole classes of Americans with whom he does not identify. He has a habit of denigrating his critics as “extremists,” even when that extremism is evinced only by his opponents’ unwillingness to sacrifice civil liberties in a bid to reengineer society along lines envisioned only by Mike Bloomberg.

Bloomberg’s effort to transform Sanders’s supporters into a liability may be incongruous with his own behavior, but it’s also a strategic imperative that has become difficult for any of Sanders’s Democratic opponents to avoid.

When Nevada’s powerful culinary union circulated literature to its members opposing Sanders’s plan to functionally nationalize the health-insurance industry, the institution met with a furious barrage of insults. The union’s leaders were called “b*tches,” “evil, entitled a**holes,” “fascist imbeciles” who deserved to “wallow in poverty and suffering” and who would soon learn that it was “time for people like me to go after you.” But the union was not intimidated. “It’s disappointing that Senator Sanders’s supporters have viciously attacked the Culinary Union and working families in Nevada,” read a statement released by the organization’s secretary-treasurer Geoconda Argüello-Kline. A victim of this harassment herself, she placed the blame for her experience and those of her union’s members squarely on Sanders’s shoulders.

The Vermont senator was soon compelled to address the brewing controversy surrounding his most vicious fans. In a statement, he urged his supporters to avoid “bullying or ugly personal attacks,” but the candidate softened this unequivocal condemnation in a subsequent appearance on PBS. “I don’t know who these ‘so-called’ supporters are,” Sanders said. “We’re living in a strange world on the Internet, and sometimes people attack people in somebody else’s name. And I’m not so sure, to be honest with you, that they are necessarily part of our movement.” He again emphasized “the nature of the Internet,” implying rather clearly that it’s not impossible that his campaign has been victimized by people pretending to sympathize with Sanders only to tarnish his reputation by association.

That wasn’t good enough for Joe Biden. “He may not be responsible for it, but he has some accountability,” the former vice president told NBC’s Chuck Todd. “You know me well enough to know if any of my supporters did that, I’d disown them,” he added. “The way they threatened these two women who are leaders in that Culinary Union. It is outrageous.”

Political analysts are prudently cautious about mistaking the online environment for the real world. They might be tempted to dismiss these efforts to make Sanders’s supporters into an election issue, but boundaries between the online world and this one have been blurred for some time. “I’m not sure Sanders’s official team has grasped the extent to which their supporters are impacting voter behavior,” observed the Daily Beast’s Sam Stein. “Lots of people at Warren and Pete town halls I talked to were weighing a Sanders vote but said they were turned off by the culture and crowds.”

While Sanders has made rhetorical overtures toward civility, he’s also surrounded himself with people who cultivate a very different atmosphere. His campaign has taken on a slate of formal surrogates who have a conspicuous habit of engaging in anti-Semitic rhetoric and who reserve the most caustic vitriol for their fellow Democrats—at least, those who do not display a sufficiently zealous commitment to his “revolution.” That is the essential nature of any campaign premised on the notion that the biggest obstacle to realizing a variety of vital policy reforms isn’t their ideological adversaries but their ostensible friends. Just as Donald Trump’s earliest supporters sought to overturn unquestioned orthodoxy within the GOP and questioned the motives of anyone who objected, Sanders has incubated a revolutionary consciousness within his movement that regards intolerance for deviationists as a virtue.

There’s nothing unfair about observing this dynamic and making Sanders answer for it. And so far, Sanders doesn’t have an answer.