WASHINGTON — President Obama launched a far-ranging attack on the Trump administration in an address to former aides on Friday, warning that the “rule of law” was endangered by the Justice Department’s decision to drop charges against former national security adviser Michael T. Flynn and criticizing the White House response to the coronavirus pandemic as “chaotic.”

“There is no precedent that anybody can find for somebody who has been charged with perjury just getting off scot-free,” Mr. Obama told hundreds of members of the Obama Alumni Association during an online discussion moderated by his friend and former White House adviser Valerie Jarrett.

Mr. Obama’s comments were first reported by Yahoo News, and confirmed by several people who heard his speech, including one who described the former president as being in “quite the mood.”

The former president, who last month endorsed Joseph R. Biden Jr., his former vice president and his party’s presumptive nominee, has signaled to people around him that he is eager to engage more directly and pointedly with President Trump in an attempt to rally Democrats around the cause of his friend — and to vanquish a man who has set out to dismantle his legacy.

The news of Attorney General William P. Barr’s decision to drop the criminal case against Mr. Flynn, who pleaded guilty to lying to the F.B.I., has been “somewhat downplayed” in the media, Mr. Obama said, during his talk with Ms. Jarrett. Mr. Obama said that such action only added to the urgency of electing Mr. Biden.

Mr. Flynn admitted that he had lied in a January 2017 interview with the F.B.I. about his contacts with Russia’s ambassador to Washington during the presidential transition. The F.B.I. was investigating whether members of Mr. Trump’s campaign might have colluded with the Kremlin to influence the 2016 presidential election.

Career prosecutors and criminal law specialists were stunned by the Justice Department’s decision to drop the case, calling it highly unusual and questioned its legal grounding. The move was recommended to Mr. Barr by a U.S. attorney in Missouri who had been tasked with reviewing Mr. Flynn’s case, after years of President Trump and his allies arguing that Mr. Flynn had been treated unfairly.

Mr. Trump has called the case against Mr. Flynn a sham, and part of an F.B.I.-led “coup” to remove him from the presidency. After being consumed for months by the coronavirus, Mr. Trump returned with a vengeance this week to his grievances over the Russia investigation, saying it was an act of treason engineered by “human scum.”

Mr. Obama has publicly adopted a studied posture of muted disapproval during his post-presidency, but privately has been caustic in his assessment of Mr. Trump’s overall performance. That was especially clear during his discussion of the administration’s response to the pandemic.

In a brief audio excerpt posted online by Yahoo, Mr. Obama said the challenge was greater than Mr. Trump.

“This election that’s coming up on every level is so important because what we’re going to be battling is not just a particular individual or a political party,” Mr. Obama said. “What we’re fighting against is these long term trends where being selfish, being tribal, being divided and seeing others as an enemy. That has become a stronger impulse in American life. And by the way we’re seeing that internationally as well.”

“It’s part of the reason why the response to this global crisis has been so anemic and spotty,” Mr. Obama continued. “It would have been bad even with the best of governments. It has been an absolute chaotic disaster when that mind-set — of ‘what’s in it for me’ and ‘to heck with everybody else’ — when that mind-set is operationalized in our government.”

Mr. Obama, who campaigned vigorously for Democratic candidates in 2018 and barnstormed for Hillary Clinton four years ago, has told friends he is deeply concerned that Mr. Trump, despite his recent stumbles, will be able to successfully leverage the bully pulpit of the presidency at a time when Mr. Biden is confined to campaigning from his basement in Delaware.