"This is a consequential moment in our history".
"This is a government for everybody, it's not for sale, that's what we believe in", Obama said.
However, he added that when there is a vacuum in democracy, "other voices fill the void".
The former president jumped into the campaign fray Saturday in Anaheim, Calif., a former Republican stronghold, to remind voters to cast ballots for Democratic congressional candidates in the upcoming midterm elections, ABC News reported.
"We have the chance to flip the House of Representatives and make sure that there are real checks and balances in Washington", he said.
Obama gave shout-outs to seven Democratic candidates in competitive House districts across California that are considered crucial to the party's efforts to oust Republicans from control. Last month, he endorsed nearly 100 Democratic candidates across the country.
During a speech at the University of IL at Urbana-Champaign on Friday, Obama said the country is in a "backlash" moment.
His comments come as Obama delivered a speech damning Donald Trump for "capitalising on resentment that politicians have been fanning for years".
Ms Clinton trounced President Trump by more than 4 million votes in California in 2016 and carried Orange County by 9 percentage points. Rep. Steve Stivers welcomed Obama's return, saying that his presence "fired up Republicans like nobody", and would unite the party in the run-up to November's contentious midterm elections. "That's on the ballot".More news: How ‘Plaid Shirt Guy’ Got Prime Seating at a Trump Rally
Obama asked, "Its central organizing principle in foreign policy was the fight against Communism, and now they're cozying up to the former head of the KGB, actively blocking legislation that would defend our elections from Russian attack".
He said stakes were high not just for Democrats but for all Americans, as "people feel scared" about a litany of issues: a troubled healthcare system, high costs of education, warming climate and more.
Although Obama didn't reference President Trump by name, he didn't need to.
Obama's campaign activity will continue through October and will include fundraising appearances, according to an adviser.
An Anaheim fire official at the scene estimated the standing-room-only crowd at the invite-only event in the convention center ballroom around 800 people.
"The biggest threat to our democracy is indifference", he said. "And I'm happy he wants to do it again".
The speech was "something different", said Esther Castaneda, 34, a small business owner from Orange, California.