But some current and former officials expressed doubt that Russian officials think that Mr. Sanders has some hidden affinity to Moscow. Instead, they said that a Russian campaign to support Mr. Sanders may ultimately be aimed at aiding Mr. Trump. Moscow could potentially consider Mr. Sanders a weaker general election opponent for Mr. Trump than a more moderate Democratic nominee, according to two people familiar with the matter.
The Washington Post first reported the briefing of the Sanders campaign. The campaign sought to pin the blame for the disclosure on the Trump administration, suggesting it was retribution for critical remarks Mr. Sanders had made about Mr. Grenell in 2018.
Russia also worked to support — or at least not harm — Mr. Sanders in 2016. Operatives at a Russian intelligence-backed troll factory were instructed to avoid attacking Mr. Sanders or Mr. Trump, according to the report by the special counsel, Robert S. Mueller III, and an indictment he secured of 13 Russians working on the operation.
Both the indictment and Mr. Mueller’s report quoted internal documents from the Internet Research Agency ordering operatives to attack Hillary Clinton’s campaign. “Use any opportunity to criticize Hillary and the rest except for Sanders and Trump — we support them,” the document said.
Russian operatives used the troll factory in 2016 to pose on social media as Americans and sow divisions among already divisive issues like immigration, religion and race. It was one part of the Kremlin’s multipronged attack on the election that also included hacks of Democratic emails, payments to unsuspecting Americans to stage pro-Trump rallies in battleground states and at least one scouting trip to the United States in 2014.
Mr. Sanders said the Russians were again trying to interfere in the campaign. Some “ugly stuff on the internet” had been attributed to his campaign that could be coming from falsified accounts, he said.
His online army of supporters is both coveted by his rivals and a source of complaints because of what they say is abusive behavior online. During the Democratic debate on Wednesday in Las Vegas, Mr. Sanders suggested that Russian trolls may be responsible for the some of the worst of the postings.
“All of us remember 2016, and what we remember is efforts by Russians and others to try to interfere in our election and divide us up,” he said. “I’m not saying that’s happening, but it would not shock me.”
Julian E. Barnes reported from Washington, and Sydney Ember from Las Vegas. Katie Rogers contributed reporting from Las Vegas, and Adam Goldman and Zach Montague from Washington.