Mr Obama has been back on the campaign trail as he bids to help fellow Democrats regain control of the House in November.
“We’re in a challenging moment because, when you look at the arc of American history, there’s always been a push and pull between those who want to go forward and those who want to look back, between those who want to divide and those are seeking to bring people together, between those who promote the politics of hope and those who exploit the politics of fear,” he said.
His appearance — one day after a strongly worded critique of Mr Trump at University of Illinois at Urbana-Champaign — touched on themes of retirement security, climate change and education.
“If we don’t step up, things can get worse,” the former president told the audience at the Anaheim Convention Centre.
“In two months, we have the chance to restore some sanity to our politics. We have the chance to flip the House of Representatives and make sure there are real checks and balances in Washington.”
Speaking about the Democrat campaign, Mr Obama said to loud cheers: “We’re going to put on our marching shoes, we’re going to start knocking on some doors, we’re going to start making some calls.”
But California Republicans said Mr Obama’s appearance would have little impact, and may even help their party.
“I wish he would come more often because he reminds Republicans of eight years of misery,” said Republican national committee member Shawn Steel.
“It reminds the Republicans why these midterms are important.”
Mr Obama is expected to deliver a similar message in another campaign speech in Cleveland on Thursday. – AP