The speech, which the former president gave after receiving an award for ethics, is Obama's first major foray into political stumping ahead of the November elections.
His appearance - one day after a strongly worded critique of Mr Trump at University of IL at Urbana-Champaign - touched on themes of retirement security, climate change and education.
The remarks were his first steps into the political fray ahead of the autumn campaign for the U.S. midterm elections.
Former President Barack Obama on Friday took direct aim at President Donald Trump for the first time since leaving office, in a speech at the University of IL in which he urged young people to participate in the 2018 midterm elections.
Obama warned that the country is at "one of those pivotal moments when everyone of us as citizens of the United States needs to determine just who it is that we are". "The stakes really are higher".
He later added: "This is not normal".
Today former President Obama gave a speech to students at the University of IL where he slammed President Trump, argued Republicans have lost their way and urged young people to vote in the 2018 midterm elections.
But, Mr Obama added, when there is a vacuum in democracy, "other voices fill the void".
Last month, Obama released a first round of endorsements - 81 candidates up and down the ballot - and there will be a second round of endorsements and additional campaign activity in advance of the midterms, according to his office.
While the former president will be visible throughout the election, the adviser said Mr Obama will not be a daily presence on the campaign trail.
Obama also said the biggest threat to USA democracy is not one individual, nor is it wealthy political supporters.
At an event in North Dakota, Trump again brought up Obama's address.
He further said that Trump rejected science, rejected facts on things like climate change, embraced a rising absolutism from a willingness to default on America's debt by not paying our bills, to a refusal to even meet, much less consider, a qualified nominee for the Supreme Court because he happened to be nominated by a Democratic President.
Obama's remarks came as he accepted the Paul H. Douglas Award for Ethics in Government from the University of IL system's Institute of Government and Public Affairs.
"For three cycles (2010, 2012, 2014) President Obama fired up Republicans like nobody", Rep. Steve Stivers, the head of the National Republican Congressional Committee, told reporters on Friday.
Also, Obama has been known to be unsuccessful in transferring votes of his supporter to other Democrats.
"That is not a check", Obama said.
Obama, who had frustrated some Democrats by keeping a relatively low profile since leaving office in January 2017, accused Republicans of being unwilling to safeguard democracy or offer a check on Trump's policies or worst instincts.
Obama argued that today's generation is the most diverse, "conditions that are ripe for exploitation by politicians who have no compunction and no shame about tapping into America's dark history of racial and ethnic and religious division".