In an unusually blistering attack on his successor, Obama said Americans were living in unsafe times and accused Republicans of threatening democracy, dividing the country, undermining global alliances and cosying up to Russian Federation.
Obama has largely avoided the spotlight since Trump succeeded him past year. Obama said: "How hard can that be? All we need are decent, honest, hard-working people who are accountable and who have America's best interests at heart".
"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".
"When you hear how great the economy's doing right now, let's just remember when this recovery started", he said, recalling that he had inherited a risky economic downturn and left office amid a recovery.
Casting aside the traditional low profile of a former president, Obama returned to the campaign trail on Friday in IL in an effort to help Democrats running in the upcoming midterm elections.
Obama said the US needs to restore "honesty and decency" in government.
Obama is seeking to rally support for Democrats as Americans prepare to head to the voting booth November 6 in elections that will see one-third of Senate seats up for grabs, as well as all seats in the House of Representatives.
The president - who was traveling in North Dakota - took a swipe back at his predecessor after the speech, telling a crowd of supporters at a fundraiser: "I watched it, but I fell asleep".
"Just a glance at recent headlines should tell you this moment really is different, the stakes really are higher, the consequences of any of us sitting on the sidelines are more dire", he said. "I found that he's very, very good for sleeping".
He later added: "This is not normal". "Or to explicitly call on the attorney general to protect members of our own party from prosecution because an election happens to be coming up".
Vice President Mike Pence says it's disappointing that former President Barack Obama is back on the campaign trail criticizing President Donald Trump.
"When we don't vote, when we take our basic rights and freedoms for granted, when we turn away and stop paying attention and stop engaging and stop believing and look for the newest diversion, the electronic versions of bread and circuses, then other voices fill the void", he said.
"We're going to put on our marching shoes, we're going to start knocking on some doors, we're going to start making some calls", he said to cheers. "Saying that Nazis are bad?"
The speech comes ahead of his first midterm campaign events, beginning Saturday, in the political battleground of Orange County, California, where he will stump for several Democratic House candidates.
Obama is expected to deliver a similar message in Cleveland on Thursday, when he campaigns on behalf of Richard Cordray, the Democratic nominee for OH governor, and other Democrats.
Next week, he plans to campaign in Ohio for Richard Cordray, the Democratic nominee for governor, and Ohio Democrats.
Obama's campaign activity will continue through October and will include fundraising appearances, according to an Obama adviser.
"In 2016, voters rejected President Obama's policies and his dismissiveness towards half the country", Republican National Committee spokesman Michael Ahrens said in response to the Friday speech. "Doubling down on that strategy won't work in 2018 either".
Obama has been playing an active role in the lead up to November's midterm elections.
Analysts say Obama's star status among Democrats will help drive turnout among African-American, Latino and young voters in key suburban House districts and cities.