Trump, the former president said, is "just capitalizing on resentments that politicians have been fanning for years".
Obama is speaking Friday at the University of IL, where he's receiving an award for ethics in government. "We've seen basic norms and principles that have served us well for decades increasingly violated", Obama said.
Obama plans to hit the campaign trail for Democratic candidates in competitive races all across the country through October, appearing first in California on Saturday and OH next week.
"Obama slammed the Republican Party's tax reform bill passed last December, which the former President argued gave "$1.5 trillion in tax cuts to people like me, who I promise don't need it".
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.
With the adulation lavished on Obama's speech in IL on Friday, where he repeatedly took shots at Trump, saying Trump's Twitter feed is nothing more than an "electronic versions of bread and circuses", insiders worry Obama's return could throw Trump off his game.
"I watched it, but I fell asleep", Trump claimed. "And the fact is that, if we don't step up, things can get worse".
President Donald Trump this week criticized the attorney general for prosecuting two Republican congressmen, saying it will hurt the party in the November elections.
Former President Barack Obama is calling on people to vote in the November election, saying "our democracy depends on it".
"Democrats need all hands on deck to take back the House, and we could not be more honored to have President Barack Obama's inspirational voice and unifying message on the campaign trail", Democratic Congressional Campaign Committee Chair Ben Ray Luján said.
In a now infamous comment following the clash of white supremacists and counter-protesters, which saw counter-protester Heather Heyer killed when a Nazi-sympathiser drove a vehicle into a crowd, Mr Trump said there were "some very fine people on both sides".
Next week, Obama plans to campaign in Ohio for Richard Cordray, the Democratic nominee for governor, and Ohio Democrats. "And, in a healthy democracy, it doesn't work", Obama said.
But California Republicans said Mr Obama's appearance would have little impact, and may even help their party. Obama was "a lot lower key than yesterday's speech".
In contrast to his speech in Illinois, Mr. Obama did not mention Mr. Trump by name, though the object of his dark warnings about the nation was obvious to his audience - who greeted him with cheers and shouts of "take it back".
Mr Obama, reacting to the article, said "that's not how our democracy is supposed to work".
Obama was editing his first public speech of the 2018 midterms on the plane, an aide to the former president tells CNN, scribbling notes and crossing out lines on a printed copy.