The speech was a break from tradition - one where former commanders-in-chief don't criticize their successors - but Obama argued that Trump's presidency represented "one of those pivotal moments when every one of us, as citizens of the United States, need to determine just who it is that we are, just what it is that we stand for".
"It's a consequential moment in our history", Obama told the crowd.
Former U.S. President Barack Obama has been traveling across the country, giving speech after speech supporting the Democratic Party as the November mid-term elections loom closer.
President Donald Trump has been a busy boy today, calling on Attorney General Jeff Sessions to investigate the author of this week's op-ed assuring Americans there is resistance inside the Trump administration.
He also said that Trump is "capitalizing on resentments that politicians have been fanning for years".
In a wide-ranging speech punctuated by frequent applause, Obama criticized the divisive politics of the era, decrying Trump's attacks on the media and the judiciary, his eagerness to work with Russian Federation while neglecting traditional alliances.
Rouda, who is seeking to unseat Rep. Dana Rohrabacher, R-Huntington Beach, said earlier that he was "very excited to have him (Obama) come to Orange County and support all these campaigns and help us flip Congress from red to blue".
Obama continued: "And when people don't participate, then that vacuum is filled by lobbyists and special interests and we get into a downward spiral where people get more and more discouraged and they think nothing's going to make a difference".
Obama is expected to deliver the same message in Cleveland on Thursday where he will campaign for Richard Cordray, the Democratic nominee for OH governor, and other Democrats. "And in a healthy democracy, it doesn't work", he added.
"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.
I would not be surprised if Obama - as long as he is in IL - does some campaigning with Democratic governor nominee J.B. Pritzker.
The midterms in November will give Americans the opportunity to "restore some sanity in our politics", Obama said.
But he also sounded a hopeful note about the apparent mobilization of Democratic voters.
In a speech last week at the University of IL at Urbana-Champaign, Mr. Obama had urged the audience to remember when the economic recovery started.
This is Obama's first campaign rally for the 2018 midterm elections.
"There is no one else in the Democratic Party that is able to focus people's attention on an issue the way that he can", said Patrick Rodenbush, the communications director for the Holder group, for which Obama recently sat to shoot a promotional video. "It's radical. It's a vision that says the protection of our power and those who back us is all that matters, even when it hurts the country". "I found he's very good - very good for sleeping", he said.
One thing, in particular, was on his mind - why exactly is it this hard for the current administration to say that Nazis are bad? "You can not sit back and wait for a savior".