By Glenn Thrush and Michael Crowley

Speaking to 3,000 supporters, the former president took a sharper line toward the Trump administration than he typically does in public.

“There is no precedent that anybody can find for somebody who has been charged with perjury just getting off scot-free,” Mr. Obama said of the Justice Department’s decision to drop charges against Michael T. Flynn. Credit…Vincent Thian/Associated Press

WASHINGTON — Former President Barack Obama said on Friday that the Justice Department’s decision to drop charges against former national security adviser Michael T. Flynn jeopardized the “rule of law,” adding his voice to the criticism of the extraordinary move.

“There is no precedent that anybody can find for somebody who has been charged with perjury just getting off scot-free,” Mr. Obama said on a call with thousands of supporters who worked in his administration or campaigns.

Attorney General William P. Barr dropped the charges against Mr. Flynn, who had pleaded guilty to lying in a January 2017 interview with the F.B.I. about his contacts with Russia’s ambassador to Washington during the presidential transition. Mr. Obama has told friends that it represented a step toward lawlessness, according to two people close to the former president, who described him as deeply angered.

Mr. Obama said the attorney general’s decision has been “somewhat downplayed” in the news media, and that it added to the urgency of electing Joseph R. Biden Jr., his former vice president and the presumptive Democratic nominee.

The former president’s comments on Mr. Flynn, which were first reported by Yahoo News and confirmed by several people who heard them, appeared to be extemporaneous and came near the end of the event, which was held in order to mobilize Mr. Obama’s supporters on behalf of Mr. Biden.

Mr. Obama and his team are seeking to mobilize 13,000 former administration officials and campaign veterans to fortify Mr. Biden’s relatively bare-bones, cash-starved campaign.

The former president spoke for about 15 to 20 minutes and at the end answered questions from those on the call. Katie Hill, a spokeswoman for Mr. Obama, said the call involved about 3,000 members of the Obama Alumni Association.

Mr. Obama devoted most of his remarks to praising Mr. Biden, recalling his service as vice president overseeing the administration’s economic stimulus program and its responses to the H1N1 and ebola outbreaks. Mr. Obama described Mr. Biden as the right person to pull together the country in a moment of crisis.

But Mr. Obama also included what one person on the call termed “very pointed” criticisms of the White House’s handling of the coronavirus pandemic.

“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, according to a brief audio excerpt posted online by Yahoo.

“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.”

Former aides stressed that the call did not signal the beginning of a more sustained public involvement by the former president, although that will clearly come later in the year.

Mr. Obama has adopted a public posture of muted disapproval of his successor during his post-presidency, although he has spoken out at moments calculated to have high impact. In the weeks before the 2018 midterm elections, Mr. Obama decried “crazy stuff” happening at the Justice Department under Mr. Trump and warned that “our democracy is at stake.”

He 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.

That anxiety has now spurred Mr. Obama to take a more active advisory role with Mr. Biden campaign, according to former aides. Mr. Obama has been in regular communication with his former vice president, discussing matters ranging from policy issues to the question of when Mr. Biden and eventually Mr. Obama can hit the campaign trail in person.

He has been especially involved in the Biden team’s effort to quickly build a strong digital operation. Mr. Obama has said he views the effort as central to competing against Mr. Trump’s small-donor fund-raising machine.

David Axelrod, who served as Mr. Obama’s senior adviser in the White House and now runs the University of Chicago Institute of Politics, said the former president was unlikely to begin speaking out more publicly about day-to-day events in the news but would become an active promoter of Mr. Biden closer to the election.

“He has really been assiduous about not being a commentator about events in Washington and I don’t think he’s going to start now,” said Mr. Axelrod, who was not on the call and is not advising any campaign. “But he’s going to be very involved in the campaign and if you look at what he did in 2017 in the governors’ races and 2018 in the midterms when he was very active, I think you see a template for what you can expect here.”

One thing that is unlikely to change is Mr. Obama’s reluctance — at least for now — to call out his successor by name.

Mr. Obama did mention Mr. Trump, albeit in passing, during 11 rallies he held in late 2018 on behalf of Democratic congressional candidates but has only sporadically done so since then. No one interviewed about his event Friday could recall hearing the president’s name except when a moderator asked Mr. Obama how Democrats could defeat Mr. Trump.

People around Mr. Obama said he preferred not to say Mr. Trump’s name, both out of disdain and a strategic desire not to get “dragged down” into a personal fight with his successor, as one former aide put it.

The practice has become so ingrained, Mr. Obama’s aides and friends have joked, that it reminds them of the Harry Potter novels in which most characters refused to even utter the name of the arch-villain Voldemort, instead referring to him as “He Who Must Not Be Named.”