But more significantly, he urged Americans to vote for Democrats in November as a check on the Trump administration's "abuses of power".
Those Democrats Obama will rally with on Saturday welcomed his return to the campaign trail. "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 nation". Trump has given Obama a chance to write a new postscript to his own legacy.
California Republicans said Obama's appearance would have little impact and may even help their party.
Last year, Democrats on Capitol Hill officially introduced a single-payer healthcare bill. "He is a symptom, not the cause".
"Because in the end, the threat to our democracy doesn't just come from Donald Trump or the current batch of Republicans in congress or the Koch brothers and their lobbyists or too much compromise from Democrats or Russian hacking", Obama said.
But in his Friday comments at the University of IL at Urbana-Champaign, the kickoff to his midterm campaign tour, Obama took a new tack, delivering a fiery speech reminiscent of the energetic oratory he used to deliver while campaigning himself.
Trump wants investigation into anonymous New York Times opinion piece
Citing company policy, Ha refused to comment on whether the Times received any substantial threats to its office or employees. "We're going to take a look at what he had, what he gave, what he's talking about, also where he is right now", he said .
"We're in a challenging moment", Obama told a crowd of hundreds at the Anaheim Convention Center. Obama was "a lot lower key than yesterday's speech".
Obama said: "If you thought elections don't matter, I hope these past two years have corrected that impression". I believe in you.
Mr Obama condemned Mr Trump's pressuring of law enforcement officials, including Attorney General Jeff Sessions.
Chelsea Brossard, Harder's campaign manager, called Saturday an "all hands on deck moment" for the campaign and said they were "proud to have this level of support as we begin the final stretch". He will spend this Saturday in California stumping for seven congressional candidates at a Democratic Congressional Campaign Committee event and will head to OH next week to campaign for gubernatorial candidate Richard Cordray.
On Saturday the former president stumped for candidates in California in the Democrats' bid to take back the House of Representatives. He said "people feel unsettled, people feel scared", and they're anxious about their children's future. "We've seen basic norms and principles that have served us well for decades increasingly violated", Obama said.
This content was republished with permission from CNN.