As virtually the entire debate stage on Wednesday turned their fire on former Mayor Michael Bloomberg, who was making his first appearance at such a forum, one candidate decided to focus on the actual frontrunner.

Former South Bend Mayor Pete Buttigieg excoriated Sen. Bernie Sanders (I-VT) for not doing enough to tamp down the behavior of his supporters online—a portion of whom have been notorious for the vitriol they direct at Sanders critics.

“At a certain point you have to ask yourself why did this pattern arise, why is especially the case with your supporters?” Buttigieg said.

Sanders, looking notably piqued by the attack, pushed back aggressively. He denounced anyone spreading hate in his name and noted that he had a lengthy record of supporting unions—even though the Culinary Union in Nevada found itself being targeted by Sanders’ supporters for its concerns about Medicare for All. And he suggested that, perhaps, the accounts that claimed to be supporters of his that were creating such havoc online may not actually be real at all but, instead, Russian bots designed to sow confusion and discord.

“All of us remember 2016, and what we remember is efforts by Russians and others to interfere in our election and divide us up,” said Sanders. “I’m not saying that’s happening, but it would not shock me.”

The social media tracking firm Graphika did identify four Russia-linked personas posing as Bernie supporters on Instagram, but the accounts, focused primarily on posting memes about American criminal justice issues, were suspended in October, before the fight over the Nevada Culinary Union.

Others are skeptical that Twitter, in particular, is home to a large Russian bot presence.

“There are not any Russian bots operating on here at the moment,” Josh Russell, a disinformation researcher, told The Daily Beast. “It's basically impossible at this point for them to operate on here. Twitter's site integrity team has made it real boring for me. I have checked and checked and checked hashtags related to the primaries, and it's just been really boring so far.”

The Buttigieg-Sanders exchange was the second of the evening between the two men who have finished atop the first two nominating states. Earlier in the evening, Buttigieg criticized Sanders as divisive, arguing that the Vermont Senator would “burn this party down.”

“We have to wake up as a party,” said Buttigieg. “The only candidates left standing will be Bernie Sanders and Mike Bloomberg. The two most polarizing figures on this stage. And most Americans don't see where they fit if they have to choose between a socialist who thinks capitalism is the root of all evil, and a billionaire that thinks money ought to be the root of all power.”

The exchange was notable in that Buttigieg seemed to recognize the reality that while Bloomberg was the shiny new object, Sanders was the one closer to running away with the Democratic Party’s nomination. Asked if he was—as Buttigieg claimed—a divisive figure, Sanders’ scoffed at the description.

“If speaking to the needs and the pain of a long neglected working class is polarizing, I think you got the wrong word,” he responded.