WASHINGTON — Russia has been trying to intervene in the Democratic primaries to aid Senator Bernie Sanders, according to people familiar with the matter, and Mr. Sanders said on Friday that intelligence officials recently briefed him.

The disclosure came a day before the Nevada caucuses, where Mr. Sanders is a favorite, and followed revelations a day earlier that Moscow was interfering on President Trump's behalf this year, as it did in 2016.

Mr. Sanders denounced Russia in a statement, calling President Vladimir V. Putin an "autocratic thug" and warning Moscow to stay out of the election. Drawing a contrast with Mr. Trump, he said he would stand against any efforts by Russia or another foreign power to interfere in the vote.

"The intelligence community is telling us they are interfering in this campaign right now in 2020," Mr. Sanders separately told reporters in Bakersfield, Calif., where he held a rally on Friday. "And what I say to Mr. Putin: 'If elected president, trust me, you are not going to be interfering in American elections.'"

Mr. Sanders said he was briefed about a month ago. Asked why the disclosure came out now, he said: "I'll let you guess about one day before the Nevada caucus. Why do you think it came out?"

On Friday, the president aggressively disputed that Russia was interfering on his behalf. He called the disclosures a hoax and part of a partisan campaign against him. At a campaign rally in Las Vegas, Mr. Trump suggested that Mr. Putin would prefer Mr. Sanders, "who honeymooned in Moscow." Mr. Sanders and his wife traveled to the Soviet Union in 1988 on a trip that political opponents have called their honeymoon, a term that the couple have jokingly used, too.

Russia's interference on behalf of both Mr. Trump, the dominant force in the Republican Party, and Mr. Sanders, a stalwart of the left, underscores its efforts to sow chaos across the political spectrum. Undermining the democratic system remains at the core of Russia's effort to raise its own stature by weakening the United States, according to current and former officials.

Russia's interference measures and their intensity remain murky, even as intelligence officials sound alarms.

In briefings to House Intelligence Committee members last week and to Mr. Sanders, officials said that Russia was actively interfering in the campaign, and people at the House briefing said intelligence officials said that Russia had a preference for Mr. Trump.

Revelations about the House briefing enraged the president, who complained that Democrats would use Moscow's support for him against him, people familiar with the matter said. Days later, he replaced the acting director of national intelligence, Joseph Maguire, though administration officials have said it was not a direct result of the briefing.

Richard Grenell, the American ambassador to Germany whom Mr. Trump appointed this week to replace Mr. Maguire, asked the agencies under his purview on Friday to provide the raw information and analysis that went into the briefing, people familiar with the matter said.

Mr. Grenell's appointment has drawn criticism from former intelligence officials who question his lack of experience and his record as a partisan ideologue. He immediately began a major personnel reshuffle, forcing out the official acting as the top deputy to the director of national intelligence.

Other officials have hastened planned exits, as Mr. Grenell looks to install his own team.

The House briefing was a supposed to be a repeat of an unremarkable classified session with the Senate Intelligence Committee in January and to inform lawmakers about a broad range of election threats, not just Russian interference. But lawmakers focused on the disclosures about Russia's support for Mr. Trump and challenged the briefer, Shelby Pierson, the nation's election security czar.

Intelligence officials disputed that Ms. Pierson said that Russia was actively aiding the re-election of the president. She did say Russia was seeking to influence American elections, including the primaries.

But people who heard the briefing said that the intelligence officers presenting the material said, in response to questions from lawmakers, that Russia was trying to get Mr. Trump re-elected.

Republicans have disputed that Russia supports Mr. Trump, insisting that Mr. Putin simply wants to broadly spread chaos and undermine the democratic system. They have also argued that Mr. Sanders's gestures of peace toward the Soviet Union at the end of the Cold War might make him appealing to Mr. Putin.

But some current and former officials expressed doubt that Russian officials think that Mr. Sanders has a hidden affinity for Moscow. Instead, they said that a Russian campaign to support Mr. Sanders might ultimately be aimed at aiding Mr. Trump. Moscow could potentially consider Mr. Sanders a weaker general election opponent for the president 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 hackings 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 race. 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 Barnes and Sydney Ember, New York Times,