Obama took direct aim at Trump for the first time in a speech at the University of Illinois in early September.
The former president said Trump was "a symptom, not the cause," of the divisiveness plaguing American politics.
"It did not start with Donald Trump," Obama said. "He is a symptom, not the cause. He's just capitalizing on resentments that politicians have been fanning for years."
Obama went on to characterize Trump as a bully and blasted the president's response to violence at a neo-Nazi rally in Charlottesville, Virginia, in August 2017. Trump controversially blamed "many sides" for the violence at the rally.
"We are Americans — we're supposed to stand up to bullies, not follow them," Obama said. "We're supposed to stand up to discrimination."
"We're supposed to stand up to discrimination, and we're sure as heck supposed to stand up clearly and unequivocally to Nazi sympathizers," Obama said. "How hard can that be, saying that Nazis are bad?"
The former president also criticized Trump's attacks on the free press. Trump routinely refers to news he doesn't like as "Fake News," and has called some of the most prominent media outlets in the country the "enemy of the people."
"It shouldn't be Democratic or Republican to say that we don't threaten the freedom of the press because they say things or publish stories we don't like," Obama said at the time. "I complained plenty about Fox News, but you never heard me threaten to shut them down, or call them 'enemies of the people.'"
In the same speech, Obama chided Republicans for taking credit for the state of the economy, contending its more positive aspects were a continuation of trends that began during his presidency.
"So when you hear how the economy is doing right now, let's just remember when this recovery started," Obama said.
This speech was part of Obama's efforts to bolster Democrats chances in the 2018 midterms. He hit the campaign trail on a number of occasions in the weeks leading up to Election Day.
During the speech, Obama told students, "You need to vote because our democracy depends on it."