In contrast to his speech in Illinois, Mr. Obama did not mention Mr. Trump by name, though the object of his dark warnings about the nation was obvious to his audience — who greeted him with cheers and shouts of “take it back.”

But from the start of his 23-minute speech, Mr. Obama, wearing a white shirt with an open collar, made clear he had set himself a different task: He was there to promote the candidacies of Democrats in California and across the country trying to win Republican seats. To that end, he named seven such Democrats running for the House, offering them brief and enthusiastic endorsements that were captured on Democratic Party cameras and that will presumably end up in candidate advertisements before long.

For the start of his support tour, Mr. Obama chose a part of the nation, Orange County, rich in political symbolism. The county was historically a conservative stronghold — the birthplace of Richard M. Nixon — that has been turning steadily Democratic over the past decade. Hillary Clinton defeated Mr. Trump here in 2016.

And five of the seven seats Democrats are going after are in Orange and San Diego Counties.

Orange County is home to the Republican that Democrats view as most vulnerable this November: Dana Rohrabacher, because of his strong support of Mr. Trump and ties to Russia. He is being challenged by Harley Rouda, a Republican turned Democrat, who was in the audience as Mr. Obama spoke.

Mr. Obama’s decision to enter the fray is not without risks. It could very well, as some Democrats acknowledge, energize Republican and conservative voters, of which there are many in this part of the state, who have long opposed Mr. Obama.