“This is where the future is heading,” said Mark Baldassare, the president of the Public Policy Institute of California. “I don’t see anything that took place in these elections or the demographic trends that are ongoing, to make me think this is a one-time incident.”

That said, the critical question for Democrats — and for Republicans eager to get back in the game — is how much of the November outcome, and the large turnout of younger Latino and Asian-American voters, was because of Mr. Trump.

Kyle Layman, who ran the Southern California congressional campaigns for the Democratic Congressional Campaign Committee, said this election had apparently begun to cement long-term changes in voter behavior — an assessment that is not disputed by California Republican leaders.

“I think what we have done is built a foundation that is going to be sustainable,” he said. “These seats are going to be swing seats moving forward. They are going to be very, very tight. But this is part of a long-term trend.”

Indeed, even if the dramatic shift on display in 2018 was in reaction to Mr. Trump — and particularly the immigration policies he has embraced — analysts said that he had only accelerated political movements that were well underway.

“Because it’s becoming more diverse it’s becoming more Democratic, because the Democratic Party is more inclusive,” said Gil Cisneros, a Democrat from Yorba Linda who captured a House seat held by Representative Ed Royce, a Republican. “This is no fluke at all. It’s been this way for a long time and it’s going to continue to trend this way for a long time.”

There was a steady decrease in white voters in the seven congressional districts that are in and around Orange County between 1980 and 2017, according to census data. In 1980, whites made up 75 percent of the population in the district where Mr. Cisneros won. By 2017, that number dropped to 30 percent.