Political and cultural transformations have swept across much of Southern California, and changed the dynamics in hard-fought races. None illustrates the changing face of Southern California’s suburbs better than the race in the 39th Congressional District, where Ms. Kim, a Republican, is running against Gil Cisneros, a Mexican-American who is seeking office for the first time. It is another of the super-tight California contests.

Asians now make up about 22 percent of the district’s population, and Ms. Kim has relied heavily on her biography: Her parents left Korea in the 1940s and raised her in Guam and Hawaii, where she often served as their translator.

“Most Democrats don’t know what to make of me,” she said. “I am a different kind of Republican — I am an immigrant. We have achieved our American dream, we see hope and possibility and positive change.”

The district straddles northern Orange County, eastern Los Angeles County and a slice of San Bernardino. All those places have become magnets for immigrants and their children to settle, buy homes and open businesses in strip malls that often evoke architecture from Asia and Latin America.

Mr. Cisneros moved to Yorba Linda, one of the wealthiest cities in the district and Richard Nixon’s birthplace, after winning a lottery jackpot. He used some of his winnings to run an education foundation, and then decided he was willing to pour millions of dollars into a campaign for Congress. His motivation, he said, came entirely after President Trump was elected.

“If Young Kim goes to Washington, she is going to be a rubber stamp for the president,” Mr. Cisneros said in an interview at his field office in Brea. “That is certainly not what this district needs or wants.”

Ms. Kim has walked a tightrope throughout the campaign. She sides with the president on some of his signature issues; she echoed his claim that the migrant caravan is dangerous, suggesting it would bring in “bad actors.” But she has opposed Mr. Trump’s family separation policy and his stances on so-called chain migration, noting that her own family and many other people in the district have benefited from family immigration sponsorships.