"This is one of those pivotal moments when every one of us as citizens of the United States need to determine just who we are, what it is that we stand for", Obama told a mostly student audience of about 1,300 at the University of IL at Urbana-Champaign.
The hourlong speech marked Obama's return to the political stage and was meant to rally Democrats ahead of November's midterm elections, which he framed as crucial for the future of the American democracy.
"In two months we have the chance to restore some semblance of sanity to our politics", Obama said. "He is a symptom, not the cause".
"Or to explicitly call on the attorney general to protect members of his own party from prosecution because an election happens to be coming up".
Trump was dismissive of Obama's speech.
Former President Barack Obama is scheduled to speak at 12 p.m. ET today.
The Times says the op-ed, published Wednesday, was written by a senior official in the Trump administration.
Mr Trump, the former president said, is "just capitalising on resentments that politicians have been fanning for years".
Tennis world erupts over stunning Serena Williams comeback
Sevastova broke the 25-year-old American's serve twice in the opening set, then broke her again to take a 2-0 lead in the second. Sloane Stephens talks dealing with the heat and being sick after her 6-2, 6-3 loss to Anastasija Sevastova in the quarterfinals.
Steve Bannon Calls David Remnick 'Gutless'
Steve Bannon's invitation to speak at a festival hosted by The Economist is still on. "Could never happen", Carrey said . But, he also noted, "I don't want well-meaning readers and staff members to think that I've ignored their concerns".
United Kingdom prime minister says no, again, to 2nd Brexit vote
The plan infuriated Brexit-backers in May's Conservative party including Johnson, who quit the government in protest. We have suffered catastrophic failure in all four engines and are losing altitude at a rate of 10,000 feet a minute.
"We're supposed to stand up to discrimination".
"By the time I left office, household income was near its all-time high, and the uninsured rate hit an all-time low, poverty rates were falling", Obama said.
"But just a glance at recent headlines should tell you that this moment really is different".
Obama said what's happening in American politics is not "conservative".
He also criticized Trump's response to the violence past year at a white nationalist rally in Charlottesville, Virginia, that resulted in the death of one person. "How hard can that be, saying that Nazis are bad?" he said.
The entire speech can be viewed below, live at Noon EST.
On Thursday, the former president is scheduled to campaign in Cleveland for Richard Cordray's campaign for Ohio Governor.
In August, Obama endorsed 81 Democratic contenders in 14 states, emphasizing young, diverse candidates running for state-level offices in an attempt to help new party leaders establish themselves.
Obama's remarks come as the midterm election enters full swing.