Former President Barack Obama speaks in Foellinger Auditorium on the University of IL campus in Urbana, Ill., on Friday, Sept. 7, 2018.
American presidents tend not to publicly criticise their successors or campaign against them directly, but Obama has been under pressure to step up to defend his policies and programmes from being dismantled by Trump and to rally Democrats who are without an impressive leader.
Obama also said the biggest threat to US democracy is not one individual, nor is it wealthy political supporters.
"This moment really is different".
"I wish he had gone a little harder on Trump", said Lee Rice, 73, an electrician from Oakdale in California's central valley. That's not how our democracy's supposed to work.
"It did not start with Donald Trump", Obama said.
Trump aides were allegedly concerned about the effect Obama's speech would have on President Trump, with Politico reporting there are fears from the Trump camp that Obama would get inside the president's head ahead of midterm elections.
For months, the 44th president has been a model of patience and restraint under a barrage of criticism and attacks -- sometimes personal and unfounded -- launched by the man who succeeded him.
'Your indifference is the biggest threat to democracy - not Trump', Obama warns
In addition, the former president said Americans concerned about Trump's presidency should be troubled by assurances contained in a recent op-ed in the New York Times from a "senior administration official" who said there was a group in the White House keeping government functioning and often working against the president.
Trump's allies in Congress were quick to make the case that Obama's Friday speech would help Trump's re-election in 2020.
Democrats, hoping to build on their 39-14 advantage in the state's congressional delegation, are eyeing Republican seats in districts that Clinton won in 2016.
Obama, who bar a handful of public appearances and an exclusive content deal with Netflix has been relatively quiet since leaving the Oval Office, accused Trump of "capitalizing on resentments that politicians have been fanning for years", during a speech at the University of IL on Friday.
"The claim that everything will turn out OK because there are people inside the White House who secretly aren't following the president's orders, that is not a check", Obama said. "The consequences of any of us sitting on the sidelines are more dire", he said.
Trump repeated his criticism of Obama's speech during a later stop in South Dakota, telling supporters: "If that doesn't get you out to vote for the midterms, nothing will".
For the start of his support tour, Mr. Obama chose a part of the nation, Orange County, rich in political symbolism.
Responding to Obama's address, Trump said he had fallen asleep listening to it. The Trump administration was under fire from Democrats for its response and handling of recovery efforts on the island.
Naomi Osaka heading a 'new generation'
Viewers also offered an outpouring of sympathy for Osaka, whose first Grand Slam win is tied up in the controversy. More voices are needed to do the same". "If I'm honest I was coaching , I don't think she looked at me", he said.