LAS VEGAS — Sen. Bernie Sanders (I-Vt.) mounted a new defense Wednesday for the toxicity emanating from pockets of his presidential campaign’s supporters, implying it was possible Russian actors were again manipulating social media to incite Democratic divisions.

Sanders’s language was indirect, offered on the debate stage here as his opponents faulted him for the behavior of his most strident fans. It drew criticism from experts in disinformation, who said they had no direct evidence the Kremlin had masqueraded as Sanders voters to interfere in the 2020 race much as Russian trolls had done four years earlier.

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

Even the insinuation that foreign actors may be to blame for controversial online behavior by Sanders’s most dedicated followers was an extraordinary indication of the extent to which election interference looms over the 2020 contest. It illustrated how few standards exist for assessing claims of meddling by Russia or other malicious actors, which are being vetted by the very same tech giants whose platforms became host to disinformation that went undetected four years ago.

The suggestion from Sanders, even though he hedged his comments, was immediately embraced by his supporters. “Absolutely,” said Tick Segerblom, a Clark County commissioner in Las Vegas who endorsed the candidate ahead of Saturday’s caucuses. “It could be Russia, or even just people trying to disrupt the system.”

He shared the senator’s view, he said, that it was “unthinkable” that his followers would go after leaders of the state’s powerful Culinary Union, given the candidate’s record of support for organized labor. The union said last week it had been “viciously attacked” by Sanders supporters for a flier issued by the group — which declined to issue an endorsement in the race — that was critical of his Medicare-for-all plan.

“Do you think I would support, or anybody [who] supports me, would be attacking union leaders? Unthinkable,” Sanders said Wednesday on the debate stage.

A spokesman for the campaign didn’t respond to a request for comment.

Absent direct evidence, researchers said Sanders’s comments threaten to foment further doubt about a campaign that has been buffeted by confidence-shaking missteps, beginning with the technical glitches that marred the Iowa caucuses earlier this month.

"We have seen no evidence in open sources during this election cycle that an online community of Sanders supporters, known as Bernie bros, were catalyzed by what Sanders suggested could be ‘Russian interference,’ " said Graham Brookie, director of the Digital Forensic Research Lab at the Atlantic Council, which tracks disinformation on social media sites.

“Any candidate or public official casually introducing the possibility of Russian influence without providing any evidence or context creates a specter of interference that makes responding to real interference harder,” Brookie said.

Facebook spokesman Andy Stone said Thursday it also had not seen evidence of Russian trolls masquerading as Sanders supporters. Twitter spokeswoman Katie Rosborough said the company would “disclose” activity by Russia or other foreign actors if it had “reasonable evidence of state-backed information operations.”

For years, Sanders has been on the defense for the aggressive tactics of his most fervent fans. During his first presidential run, aides to the senator’s campaign privately expressed embarrassment about the online harassment that seemed to be coming from “Bernie bros,” according to two advisers on the campaign who spoke on the condition of anonymity to describe private conversations. But there was also confusion about who exactly those supporters were, the sources said.

Aides used to joke about Twitter eggs — the default image for new accounts — popping up to attack people, creating a fresh headache for the campaign. Although some doubted the users were authentic supporters, the advisers said, there was no consideration at the time that some of the activity might have been fomented by deceptive Russian agents, as a U.S. investigation of election meddling would later reveal.

In 2020, the campaign’s supporters have been similarly vocal. In response, aides have labored to redirect the energy, rather than reprimanding some of the worst offenders outright. “Rage donating is better than rage tweeting,” one organizer for the campaign recently wrote. “Pass it on.”

But the debate Wednesday brought new attention to the question of accountability for attacks lobbed in a candidate’s name.

“We have over 10.6 million people on Twitter, and 99 percent of them are decent human beings, are working people, are people who believe in justice, compassion and love,” Sanders said. “And if there are a few people who make ugly remarks . . . I disown those people.”

He also said his campaign bore no more blame than other factions, even though critics say the vehemence of his online following is unmatched among his Democratic rivals. Some of his rivals sought to press the point on the debate stage. “I think you have to accept some responsibility and ask yourself what it is about your campaign in particular that seems to be motivating this behavior more than others,” said Pete Buttigieg, a former mayor of South Bend, Ind.

The senator’s invocation of Russian interference seemed designed to cast doubt on the question of motivation.

Experts, however, cautioned against a rush to attribute the actions of Sanders’s supporters to the same malicious foreign actors that disrupted the 2016 race.

“The problem we face is the impression of meddling can be as damaging to the legitimacy of an election as meddling itself,” said Jessica Brandt, the head of policy and research at the Alliance for Securing Democracy, an arm of the German Marshall Fund.

So far, Brandt said, the primary driver of disinformation online is "Russian government-funded media, and their diplomatic Twitter accounts.” The Russia-backed media sites RT and Sputnik, for example, have taken to Twitter to advance the idea that the Democratic National Committee has engaged in "dirty tricks" to undermine Sanders, according to her organization’s research. Weeks later, some of those same accounts pivoted to riling up "existing divisions among Democrats," the group found.

To that end, Brandt said there is "no evidence I've seen covert accounts are masquerading as Bernie supporters."

In October, Facebook announced it had disabled a network of fake accounts that attacked former vice president Joe Biden and praised candidates including Sanders and President Trump. But the campaign, which aimed to sow division among Democrats, largely repurposed existing memes from American social media users, according to Graphika, which studied the Facebook data.

Researchers have found some of the most divisive tweets and memes during the 2020 election originate with specific, verified Sanders supporters. That includes the viral #mayorcheat hashtag that attacked Buttigieg for ties between the former mayor’s presidential campaign and Iowa caucus organizers, said Ben Nimmo, director of investigations at Graphika.

“Russian operations have a track record of amplifying American divisions, but I’ve not seen any evidence to suggest that the Bernie bro phenomenon is the result of Russian interference,” Nimmo said.

Online discourse has become so polarized this election cycle that “you don’t need Russians to inflame it,” he said, adding: “Americans are doing it to each other. And saying that it’s caused by Russian operators without presenting any evidence just makes it harder to address either problem — American polarization or genuine Russian interference.”