Barack Obama has accused Donald Trump of “appealing to fear” in a stinging attack.
In a break from tradition for a former US president, most of whom remain quiet on subsequent leaders, Mr Obama accused his successor of “capitalising on resentments that politicians have been fanning for years”.
His damning words were delivered less than two months before midterm elections that could determine the course of Mr Trump's presidency.
Mr Trump responded by claiming he fell asleep watching Mr Obama's speech.
"Appealing to tribe, appealing to fear, pitting one group against another, telling people that order and security will be restored if it weren't for those who don't look like us or don't sound like us or don't pray like we do - that's an old playbook," Mr Obama said at the University of Illinois on Friday.
“It's as old as time. And in a healthy democracy, it doesn't work. Our antibodies kick in and people of good will from across the political spectrum call out the bigots and the fear-mongers and work to compromise and get things done and promote the better angels of our nature."
But he added, when there is a vacuum in democracy, "other voices fill the void. A politics of fear and resentment and retrenchment takes hold".
Mr Trump shrugged off the attack, claiming that Mr Obama is “very good for sleeping” at a campaign appearance in Fargo, North Dakota.
He also said the former president was trying to take credit for this "incredible thing that's happening to our country".
Even as he has largely remained out of the spotlight, Mr Obama made it clear he has paid close attention to the steady stream of headlines chronicling the Trump administration.
"Just a glance at recent headlines should tell you this moment really is different," he said.
"The stakes really are higher. The consequences of any of us sitting on the sidelines are more dire."
He later added: "This is not normal."
He also Mr Trump's record of putting pressure on law enforcement officials, including attorney general Jeff Sessions and jabbed him on the improving economy
"When you hear how great the economy's doing right now, let's just remember when this recovery started," he said.
