WASHINGTON — President Trump on Tuesday tried to diminish the significance of a former foreign policy adviser, who admitted to lying to the F.B.I. about how, during last year’s presidential campaign, he sought to meet with Russians offering “dirt” on Hillary Clinton based on purloined emails.

In his first comment on this aspect of the case being developed by the special counsel investigation, Mr. Trump did not deny that the foreign policy adviser, George Papadopoulos, worked to collaborate with Russia. He simply brushed off his significance and focused on the fact that Mr. Papadopoulos pleaded guilty to lying to the F.B.I. to cover up the contacts with Russia.

“Few people knew the young, low level volunteer named George, who has already proven to be a liar,” Mr. Trump wrote on Twitter.

As for the indictment of his former campaign chairman, Paul Manafort, the president repeated that the crimes alleged took place outside the context of the election contest.

The Fake News is working overtime. As Paul Manaforts lawyer said, there was “no collusion” and events mentioned took place long before he…

— Donald J. Trump (@realDonaldTrump) October 31, 2017

….came to the campaign. Few people knew the young, low level volunteer named George, who has already proven to be a liar. Check the DEMS!

— Donald J. Trump (@realDonaldTrump) October 31, 2017

As he has repeatedly in recent days, Mr. Trump sought to turn attention to Democrats, pointing to the resignation of Tony Podesta, the powerhouse Democratic lobbyist who also faces scrutiny by Mr. Mueller and whose brother, John D. Podesta, was Mrs. Clinton’s campaign chairman.

“The biggest story yesterday, the one that has the Dems in a dither, is Podesta running from his firm,” Mr. Trump wrote. “What he know about Crooked Dems is earth shattering. He and his brother could Drain The Swamp, which would be yet another campaign promise fulfilled. Fake News weak!”

Mr. Manafort and his deputy, Rick Gates, were charged in a 12-count indictment with a series of money laundering, tax evasion and foreign lobbying crimes stemming from work for pro-Russian political leaders in Ukraine. While the crimes alleged began years before Mr. Trump’s campaign, the indictment asserted that Mr. Manafort’s scheme to defraud continued through last year until early this year.

Mr. Papadopoulos was named by Mr. Trump in March 2016 as one of five foreign policy advisers. While the president and his team now seek to minimize Mr. Papadopoulos’s importance, at the time Mr. Trump described him in flattering terms. “He’s an energy and oil consultant, excellent guy,” he told The Washington Post.

According to a statement of offense signed as part of his guilty plea, Mr. Papadopoulos admitted that he spent months last year cultivating contacts in an effort to arrange meetings between Mr. Trump’s campaign and Russian government officials.

Mr. Papadopoulos said a London-based professor with extensive Russian contacts introduced him to a woman described as “Putin’s niece” and told him the Russians had “dirt” on Mrs. Clinton based on “thousands of emails” of hers. (The woman was not actually related to President Vladimir V. Putin.)

The professor, identified on Monday by a Senate aide as Joseph Mifsud, told Mr. Papadopoulos about the emails in April 2016, three months before WikiLeaks released nearly 20,000 hacked Democratic emails. Mr. Papadopoulos kept senior campaign officials informed about his efforts and they encouraged him but made clear they wanted to keep some distance publicly. “It should be someone low level in the campaign so as not to send any signal,” a top campaign official wrote in an email at the time.

Mr. Papadopoulos was not charged with any crime for making those efforts but instead pleaded guilty to lying to F.B.I. agents about the matter. He was arrested secretly in late July and has been cooperating ever since with the team of the special counsel, Robert S. Mueller III.

“George was a low-level volunteer who might have attended a meeting of the foreign policy advisory team, the one meeting that took place but he was not a person who was involved with the day-to-day operations of the campaign, or a person who I recall interacting with on a regular basis at all,” Corey Lewandowski, who ran the campaign before Mr. Manafort, said on the “Today” show on NBC on Tuesday.

Mr. Lewandowski said he did not learn about the hacked Democratic emails until they became public. “To the best of my knowledge, absolutely not,” he said. “When I found out about that, I found out about it through public press reports.” He added that he has not spoken with the F.B.I. but would be “happy to do that unequivocally.”

Mr. Papadopoulos’s efforts are the second known effort by a member of Mr. Trump’s team to obtain damaging information about Mrs. Clinton from the Russians. Several weeks after the meeting where Mr. Papadopoulos learned about the Clinton emails, Donald Trump Jr., Mr. Manafort and Jared Kushner, the future president’s son-in-law, met with a Russian lawyer after being promised incriminating information about Mrs. Clinton from the Russian government.

They said later that the meeting did not yield such information, but the president has defended their decision to take the meeting as a routine opposition research. The original statement to The New York Times by Donald Trump Jr. describing the meeting, crafted with the participation of the president, omitted the promise of damaging information. Mr. Mueller is now looking into that statement.

In Moscow’s initial reaction to Mr. Mueller’s indictments, Russian state-run news media and government representatives on Tuesday emphasized that the accusations laid out against Mr. Manafort did not mention Russia. Instead, media reports highlighted that he was accused for his actions in Ukraine. But his work in Ukraine was for the pro-Russian party of President Viktor F. Yanukovych, who was driven out of office by street protests in 2014.

Dmitri S. Peskov, the Kremlin’s spokesman, called all accusations of Russia’s involvement in last year’s election “laughable, unqualified, groundless and unsubstantiated.”

“From what we have read in the media and seen in the statements by participants of this process, so far Russia was not mentioned in any way in the accusations, other countries were,” Mr. Peskov said at his regular news briefing.

“Moscow has never felt guilty to feel vindicated now,” Mr. Peskov said. “We were always puzzled about these groundless and unsubstantiated accusations of Russia’s alleged involvement in American elections,” he said. “We have resolutely denied them from the beginning and we deny them now.”

American social media companies told Congress on Monday that Russia was prolific in its efforts to reach American voters last year to sow discord. Russian agents published inflammatory posts that reached 126 million users on Facebook, published more than 131,000 messages on Twitter and uploaded over 1,000 videos to Google’s YouTube service, according to prepared remarks obtained by The Times.