Like many Republicans in competitive districts, Ms. Kim has so far stayed silent on the impeachment inquiry and declined an interview request.

President Trump is despised among much of California, but he remains popular among many Republicans in the state. And Ms. Kim has walked a careful tightrope: She frequently distanced herself from the president during the 2018 campaign, though she now says his record is “pretty good.”

“A lot of people agree that the economy is the number one issue for them, and Donald Trump is doing the right thing in this area,” Ms. Kim said, adding that many voters in the district tell her they support his efforts to dismantle many federal regulations. “That’s a key issue for all voters and especially women,” she said. “We’re going to need to focus on the results.”

The district straddles northern Orange County, eastern Los Angeles county and a slice of San Bernardino — suburban areas that have become popular places for immigrants and their children to settle. Demographically, the area has transformed: The district is roughly evenly split between Asian-Americans, Latinos and whites. Like Ms. Kim, many were drawn to the area because of its schools and newer homes, and their influence is clear at strip malls all around the region: Ms. Kim’s last campaign office was flanked by Taiwanese and Oaxacan restaurants .

“National issues here are local issues,” said Mark Baldassare, the president of the Public Policy Institute of California, which released an extensive survey Wednesday showing that Mr. Trump’s approval rating hovered around 30 percent in the region. “With demographic changes have come changes in attitude, not just about immigration but also on issues like the environment and taxes.”

For years, Ms. Kim was a district aide to Congressman Ed Royce , and she decided to run almost immediately after his retirement announcement. But with an open primary, there was no guarantee that a Republican would advance to the general election and the national party initially stayed out of the race.

As Democrats rallied to capture several seats in Southern California during the midterms, many Republicans saw the brightest star in Ms. Kim, who routinely mentions her experiences as a Korean-American immigrant and mother of four. Ms. Kim’s candidacy could be traced to Republican efforts after the 2012 election to recruit more candidates of color.