With little time to reverse the course of the Democratic primary, leftist and independent-minded critics of Joe Biden have ramped up insinuations about the former vice president’s mental state, going so far as to insist that the matter will doom the party in the general election.

The attacks are remarkable not just for the degree they copy the current Republican playbook against Biden but for how they take a topic that was taboo in past cycles and—without any verification—make it a centerpiece of efforts to blunt his momentum. Coming after the former VP wrested the lead from Sen. Bernie Sanders (I-VT) following the primaries on Super Tuesday, it suggests that the closing chapter of the Democratic primary could turn into a deeply personal and politically nasty affair.

Indeed, some Sanders supporters have been blunt about their desire to gin up chatter about Biden’s mental state to derail him. In an episode of the popular leftist podcast Chapo Traphouse released Thursday, the hosts openly discussed how best to use the “senility” attack against the former vice president.

“I’m sorry, he’s senile... his brain is not all there,” Chapo co-host Will Menaker said.

Fellow co-host Amber A’Lee Frost urged Sanders supporters to use questions about Biden’s mental state selectively, raising questions about Biden’s support for Social Security when dealing with older voters instead.

“Now we go negative, but I think there’s going to have to be a two-front thing,” she said. “Because the ‘Biden is crackers’ thing isn’t going to work on his base, who are themselves also crackers.”

On Sunday, Sanders himself was asked if he felt Biden was “up to the task in terms of the rigors of being either the Democratic nominee or being the president.” The Vermont senator didn’t endorse the idea that Biden was physically incapable of the job. But he didn’t aggressively beat down the chatter either. “Joe Biden is a friend of mine,” said Sanders, in an appearance on CNN’s State of the Union. “And Joe and I have disagreements on the issue. I do not make personal attacks on Joe.”

“ I do not make personal attacks on Joe. ” — Bernie Sanders

One day earlier, the hashtag WhereIsJoe began trending on Twitter as Biden critics pushed the idea that the campaign was putting him out of sight in order to hide evidence of his supposed cognitive decline. It was a curious effort, not just because there is no direct evidence to support the charge but because Biden had events throughout the weekend. That, however, hasn’t stopped the attacks from spreading.

“Biden is either near senile or actually senile,” Cenk Uygur, the founder of left-wing online news outlet The Young Turks, said in a video after Biden upended the race on Super Tuesday (and as Uygur reeled from his own loss in a House primary). “Watch any of the tapes! His mental faculties are under question.”

Intercept Washington bureau chief Ryan Grim highlighted a clip of Fox News host Sean Hannity calling Biden “completely incapable of remembering basic fundamental facts” and asked how Biden would handle questions about his mental stability if he wins the nomination. Outspoken Biden critic and pugnacious economic populist Matt Stoller claimed that Biden’s mental decline is an open secret in Washington.

“They joke about it,” Stoller wrote of Washington insiders. “They don’t care.”

And as the weekend progressed, several others got in on the act, including The Intercept’s Glenn Greenwald and 2016 Green Party candidate Jill Stein.

Prior to the 2016 election, speculation about the mental health of a presidential candidate was frowned upon, as anyone doing so would be operating without primary knowledge of the subject matter. That changed when Donald Trump began having success on the campaign trail, prompting his opponents, and some medical professionals, to abandon the informal rule against such speculation.

The Biden campaign didn’t respond to a request for comment. But there is no documentation that Biden’s critics can point to to base their accusations of a diminished mental state. In December, the campaign released medical records that described Biden as “fit to successfully execute the duties of the Presidency.” The medical records didn’t describe Biden as suffering from any mental issues.

But Sanders’ supporters have keyed in Biden’s seemingly limitless capacity for producing embarrassing moments on the campaign trail as a sign that Biden faces a unique liability in the general election and may not even be physically capable of running. Just last week, Biden garbled a quotation from the Declaration of Independence and botched the introductions of his sister and wife in his Super Tuesday victory speech. Ahead of his South Carolina win, Biden told a crowd, wrongly, that he was running for the Senate.

It has not been uncommon for Fox News and other pro-Trump media outlets, as well as Trump and his family members too, to jump on these often uncomfortable clips. But now they’ve been joined by more left-leaning critics of the former VP, the more direct of whom use them to question Biden’s capacity for the job, the less direct of whom argue that they will be used against him in the same way that video of Hillary Clinton falling at a 9/11 event was used against her in the 2016 campaign.

The debate about Biden’s mental state recalls the years-long discussion about Trump’s own mental state, which has been fueled by his own garbled remarks at rallies and insiders’ bizarre descriptions of his behavior. Those discussions brought renewed attention to the American Psychiatric Association’s “Goldwater Rule,” which discourages members from publicly speculating about the health conditions of public figures from afar.

Still, not everyone is willing to follow the Goldwater Rule. Psychologist John Gartner, who founded a PAC calling for Trump’s removal via the 25th Amendment on grounds of mental unfitness, said he doesn’t think Biden is suffering from mental deterioration. Instead, Gartner attributes Biden’s gaffes on the debate stage to Biden’s struggle with stuttering.

“He knew that the clock’s ticking and he’s not completing the sentence, he’s not completing the thought,” Gartner said.

Because Biden has struggled with public appearances for decades, Gartner said, his most recent round of embarrassing clips don’t necessarily signify mental decline.

“I’m not seeing any dramatic, clinical signs,” Gartner said.

Complicating the leftist attacks against Biden’s health is that Sanders faces a round of health-related questions and speculation of his own. The senator survived a heart attack in October. And despite once committing to releasing comprehensive medical records, the Sanders campaign still hasn’t disclosed all of the senator’s medical information. Instead, the campaign released three letters from doctors who examined Sanders after his heart attack, with one doctor writing that Sanders has “the mental and physical stamina” to be president.