Irvine, California (CNN) If recent population trends continue at their current pace, it's likely that by Election Day this November, minorities will constitute the majority of the population in the Congressional District centered on this prospering small city in Orange County about an hour south of Los Angeles.

That places the 45th Congressional District of California, whose seat is now held by Republican Rep. Mimi Walters, at the forefront of a trend that's steadily reshaping the landscape of House seats across the country: the inexorable growth of racial and ethnic diversity.

A CNN analysis of census data found that from 2010 through 2016, the latest year for which figures are available, the minority share of the population increased in all but 24 of the 435 congressional districts. In 123 districts, the minority population increased over that period by at least 3 percentage points, or half a percentage point annually.

The result was that the number of districts where minorities represent at least 40 percent of the population increased by 22 just from 2010 through 2016, reaching 171 in all. That's nearly 2 of every 5 districts and that number is certain to crest even higher by the next census in 2020.

"That's the demographic tide that is rising," says Douglas Massey, a Princeton University sociologist who studies population patterns.

Orange County exemplifies as clearly as any place in the country both the opportunities created for Democrats by increasing diversity and the obstacles that might still frustrate them. Now the sixth largest county in America, with over 3 million residents, it sprawls from breezy beachfront mansions in Newport Beach and Laguna Niguel to gritty working-class neighborhoods in Santa Ana and Fullerton.

For decades, Orange County sent conservative firebrands such as Robert Dornan and William Dannemeyer to the House and was so reliably red that President Ronald Reagan famously joked it was where all good Republicans went to die. After Franklin Roosevelt carried the county in his landslide first re-election in 1936, no Democratic presidential candidate won it again until 2016, when Hillary Clinton narrowly beat Donald Trump there.

Now Democrats are mobilizing to mount their most serious challenge yet against Walters, a low-key former mayor of coastal Laguna Niguel and state legislator first elected to Congress in 2014. Democrats are also pressing hard for the neighboring Orange County seat held by Republican Dana Rohrabacher, as well as the open seats being vacated by GOP Reps. Ed Royce and Darrell Issa, who are retiring.

Demographic change has keyed that political transition. When Reagan was first elected President in 1980, whites still composed nearly 80% of the county's residents, with Hispanics representing 15% and Asians just 4%. By 2010, whites had fallen to 44% of the population, with Hispanics soaring to 34% and Asians spiking to 18%. (African-Americans, at 3%, and mixed-race residents, at 1%, remain small communities.)

The demographic wave has continued to roll since. Just since 2010, the CNN analysis found, the nonwhite share of the population increased by nearly 6 percentage points in Royce's district (rising past 70% in all) and almost 5 points in Walters' district. That put the white share of her district at just 50.8% in 2016; given the underlying rate of minority growth (just under 1% annually), that means it's likely that whites will fall below majority status there by November (if they haven't already).

Orange County is now the kind of place where the young professionals who gather for lunch and after work in the stylish 4th Street Market in Santa Ana can order half a dozen varieties of Mexican michelada drinks, exotic hand-crafted ice cream pops (sample flavor: mango-chile) or a "lemongrass burrito" -- the perfect symbol of how Asian and Mexican cultures are jointly transforming this once sleepy suburb. The question is whether the politics can keep up with the cuisine.

Watching Orange County change before his eyes

Gustavo Arellano sitting at the 4th Street Market in Santa Ana.

Gustavo Arellano, a sharp-eyed local writer and former editor of the OC Weekly, sees historic irony in that prospect. The city of Irvine, the centerpiece of Walters' district, was developed as one of the nation's largest planned communities in the late 1960s by the Irvine Co., headed by Donald Bren, a prominent Republican and staunch Reagan ally.

"Irvine used to be the epitome of 'Stepford Wives' whiteness in Orange County," Arellano says, sitting in a stylish grocery and deli run by his wife in Santa Ana's bustling 4th Street Market. "The kind of place where you could paint your house any color as long as it's beige. But that's been for years now a minority majority city."

Arellano has watched Orange County's transformation firsthand. He grew up in Anaheim, with a mother who was a legal US resident and a father who was an undocumented immigrant (who crossed the border through means ranging from hiding in the trunk of a Chevrolet driven by a "hippie chick from Huntington Beach" to hiking through the desert).

Arellano remembers a school field trip that, in retrospect, seems like a glimpse into the future.

"When I was in kindergarten we went to the beach with senior high schoolers," he says. "I just remember those seniors were all white, and all us ... kids in kindergarten in Anaheim were all Mexican."

The steady increase in the Hispanic population has concentrated in the county's north central areas, such as Santa Ana, Anaheim, Fullerton and Buena Park. While those communities still face significant obstacles in earnings and education, the share of younger Hispanics from the area finishing high school and attending college is growing, notes Jody Agius Vallejo, a University of Southern California sociologist who's the author of "Barrios to Burbs: The Making of the Mexican American Middle Class."

"There is this cohort that is coming of age, and is really poised to enter into the middle class and do much better than their parents," Vallejo says. That economic advance, she notes, will also provide the foundation for greater political influence.

A mural at the 4th Street Market in Santa Ana.

The growth of Orange County's Asian-American population began with South Vietnamese refugees who settled in "Little Saigon" near Westminster in the mid-1970s; today over 200,000 Vietnamese-Americans live in Orange County, the largest concentration in the country, according to "Transforming Orange County," a recent report from the civil rights group Asian Americans Advancing Justice.

They have been joined in the county by a panorama of other Asian-American communities: over 94,000 Korean-Americans, nearly 90,000 Filipino-Americans, 84,000 Chinese-Americans (plus another 14,000 from Taiwan) and nearly 50,000 Japanese-Americans. Retail stores, ethnic restaurants, high-tech start-up companies and even the giant Asian-themed Diamond Jamboree shopping mall in Irvine all reflect the imprint of this growing presence.

Inexorably, the growing Hispanic and Asian populations have left their marks on politics too. Hispanics have broken solidly toward the Democratic Party: Loretta Sanchez, a Democratic US representative, became renowned as a giant killer for beating the flamboyantly conservative Bob Dornan in a district centered on Santa Ana back in 1996.

A shift of Asian-Americans away from the GOP

But Orange County's Asian-American community has tilted consistently toward the Republican Party. That orientation began with the staunchly anti-communist Vietnamese community in the 1970s. Republicans have sustained the advantage since, even as Asian-Americans have moved sharply toward the Democrats elsewhere around the country in recent years.

"If you look [in Orange County] at the city councils, the mayors, to school boards, those Asian-Americans for the most part have been Republicans, and Republicans have done more outreach," says Linda Trinh Vo, a professor of Asian-American studies at the University of California, Irvine. "And because Orange County is so Republican, that's how they thought they could win. So the two kind of reinforced one another."

Lisa Bartlett is an Orange County Supervisor and a Republican.

Today Asian-Americans hold three of the five seats on the Orange County Board of Supervisors. Strikingly, all three of them are Republicans.

"The Orange County GOP has really done an outstanding job on outreach into the Asian-American communities," says Lisa Bartlett, a Japanese-American Republican who serves on the board. "Getting people to be more engaged in their own communities and helping them run for office, at the local level ... and moving up to county, and state and federal as well."

Long beleaguered, local Democrats believe the county's dynamics are shifting under Trump -- as exemplified by Clinton's surprising win here in 2016. (She carried all four of the Republican-held congressional districts in the county.) As in many white-collar suburbs, local polls show college-educated whites cool toward Trump. And many local observers believe Trump's racially infused nationalism, punctuated by his recent attempt to reduce not only undocumented but also legal immigration, could prompt both a large turnout among Hispanics and new openings for Democrats with younger Asian-Americans not as firmly tied to the GOP as older generations.

"There is that generational divide in terms of the parties," says Vo. "[Democrats] still have a lot of work to do in building that base. But I think there's the potential now to get these individuals elected."

A Trump effect, but a shallow bench

David Min is running for Congress as a Democrat.

David Min is betting she's right. Min, a former aide to Democratic Sen. Chuck Schumer of New York and a law professor at UC Irvine, is the son of Korean immigrants who settled in Orange County during the 1970s. Now, behind a grass-roots campaign that's seen him knock on 10,000 doors since early last year, he's a leading contender for the Democratic nomination to oppose Walters. He believes unease with Trump is creating openings both with white independents and Asian-Americans who have traditionally leaned Republican.

"I think a lot of the folks here you might describe as fiscally conservative," Min told me recently. "At the same time they are not really Trump Republicans. They voted for Mitt Romney at a heavy clip. But these folks are not anti- immigration, they are not anti-education, they are not anti-science. So I think there is a real chance to win over these types of voters."

Min embodies the energy in the Democratic opposition to Trump. He decided to run because he was infuriated by the President's executive order early last year banning travel from several majority-Muslim nations.

"It felt very un-American, contrary to everything I think this country stands for," Min says, "the idea that we're going to target [people] based on where they come from or the God they worship."

He is unsparing in his criticism of Trump's recent efforts to reduce legal immigration. A town hall Min held here in an industrial park on a Monday night drew an overflow crowd of enthusiastic Democrats who peppered him with questions about his strategy for uniting the party and beating Walters.

And yet Min, who has never previously sought office, acknowledges that he might not be running at all if Democrats had more local Orange County officials who might plausibly step up to challenge Walters. Across all the contested Orange County seats, in fact, almost all the leading Democratic contenders have never held public office. Meanwhile, Republicans have a deeper bench of local officeholders, with their own networks of supporters and donors, lining up to seek the two open seats in the district. That includes Korean-American Young Kim, a former California state assemblywoman and Royce staffer, who is seeking his seat, and Hispanic state Assemblyman Rocky Chavez, who is running in Issa's district.

Largely because Democrats are relying on such untested candidates, Arellano is notably bearish on their prospects everywhere in Orange County except the seat Issa is vacating.

"There is no name recognition," he says. "There is no support system. Democrats for far too long have relied on demographics to win elections."

Population change 'is like an IV drip'

Population change almost never occurs rapidly enough to immediately shift the partisan advantage in a district from one party to another. But, like a river cutting through rock, growing diversity is combining with other changes, particularly shifting allegiance among white-collar white voters, to create new opportunities for Democrats, particularly in Republican-held House seats around major metropolitan areas.

Population change "is like an IV drip; it is baby steps and it takes time," says David Jacobson, a California-based Democratic consultant. "But what we have seen with the Trump phenomenon is like nothing we've seen before."

Democrats still face serious obstacles in converting growing diversity into improving electoral chances this fall. During midterm elections, turnout among nonwhite voters usually sags more than it does among whites. In some of the diversifying districts, Republicans have established a consistent record of performing better with nonwhite voters than they do nationally -- Latinos in parts of Texas, for instance, or Asian-Americans here in Orange County.

"The trends are all going positive for more minorities in these areas, but how many of them are citizens, how many are eligible to vote, how many are adults?" says Patrick Ruffini, a Republican consultant who studies demographic change. "That demographic growth is going to manifest itself over time in a big way. But for today, this cycle, what does that mean?"

Though it's rarely discussed, the racial composition of congressional districts represents one of the starkest separations between the two parties in the House.

To understand the intertwined patterns of racial change and partisan preference, CNN data analyst Ryan Struyk compared the racial composition of congressional districts in 2010 (using data from the 2010 decennial census that the Census Bureau has reconfigured to the congressional district lines first used in 2012) with the 2016 results of the census' annual American Community Survey (the most recent available).

That analysis found the minority share of the population increased over those six years in a wide array of districts in all parts of the country. That population grew by 5 percentage points or more in 22 districts; between 4 and 5 points in 34 districts; from 3 to 4 points in 67 districts; from 2 to 3 points in 112 others; and by less than 2 percentage points in 176 more. It declined in just 24 districts.

Even amid the growing dispersal of minorities, though, diversity remains unevenly distributed across the US.

The white share of the population still exceeds its 61.1% national average (as of 2016) in 252 congressional districts (including three with seats that are currently vacant). Republicans hold 187 of the seats that are not vacant, or fully 75%. In 183 districts, the minority share of the population exceeds the national average (including one seat that is vacant). Democrats hold 131 of those occupied seats, a comparable 72%.

Seen from the other angle, nearly four-fifths of House Republicans hold seats where the white share of the population exceeds the national average. By contrast, over two-thirds of Democrats hold seats where the white population share lags the national average.

Some of the most vulnerable members on either side are those clinging to seats that defy these demographic trends. Whites now make up at least 80% of the population in 108 districts. Republicans already hold 88 of them and Democrats control just 18. (Two are vacant.) Most of those preponderantly white Democratic-held seats are in New England and many of them remain safe for the party. But that list also includes Democratic seats in Minnesota, Wisconsin, New Hampshire and Pennsylvania that Republicans are hoping to turn this fall, particularly given Trump's strength among white voters without a college education.

Conversely, Republicans hold just 21 of the 123 seats where minorities represented a majority of the population as of 2016 -- including nine in California, seven in Texas and three in Florida. Most of the Texas-based majority-minority seats appear safe for the GOP, largely because they run more competitively among Hispanics than Democrats do among whites there. But Democrats are targeting many of the seats on this list -- including the suburban seats held by Republicans John Culberson near Houston, Pete Sessions near Dallas, Rob Woodall near Atlanta, Carlos Curbelo in Miami, the Central Valley of California seat of Jeff Denham and the district that Royce is vacating in Orange County.

In all, analysts in both parties, and outside handicappers, give Democrats a serious chance in about half of the 20 Republican-held seats that have experienced the biggest minority population growth since 2010. That list includes many of the Republican-held majority minority districts as well as several others where whites retain the majority despite that growth, such as the Northern Virginia seat held by Barbara Comstock and the seats being vacated by Charlie Dent in Pennsylvania, Dave Reichert in Washington state and Rodney Frelinghuysen in New Jersey. In such closely contested districts, a slight increase in the minority share of the vote could be a tipping point thumb on the scales for Democrats.

Bigger changes coming

Whatever happens in 2018, the biggest impact of the dispersing minority population may be felt after the redistricting that follows the 2020 census. Manuel Pastor, director of the Program for Environmental and Regional Equity at USC, notes that Asian-American and especially Hispanic political influence may increase at a faster rate in the next several years because their population growth is now less dependent on new immigrants, who must wait years to become citizens eligible to vote, and more on young people who are born US citizens. The diffusion of the minority population, he argues, may also increase the number of white voters who recoil from Trump's harsh tone toward immigrants.

"Because when those white voters start to have more moms and dads in the soccer club who are south Asian or Latino or immigrant, they tend to be a little more offended by the kind of comments that Trump is putting out," Pastor says.

As minorities account for more of the total population growth, particularly in Sun Belt states such as Texas, Arizona and Florida, it will also grow more difficult for even Republican-controlled legislatures to draw districts that dilute their voting power. In Texas, for instance, Democratic consultant James Aldrete says that after "another decade of minority [population] growth outpacing Anglo growth it is tricky on how you are going to ... deny that" in allocating state legislative and congressional seats.

The tension is that these minority populations are growing and dispersing even as the President has demonstrated how many whites will respond to political appeals that touch, directly or indirectly, on anxiety about precisely that demographic change. Though Trump's proposals to reduce legal immigration by about 40% were soundly defeated during the recent Senate debate, for instance, the vote still represented a marked advance for anti-immigration arguments in the GOP. In 1996, the last time the Senate voted on reducing legal immigration, about three-fourths of GOP senators voted no; this time, about three-fourths of GOP senators supported the cuts in legal immigration.

Massey, the Princeton sociologist, says history shows a consistent pattern to such anti-immigration movements. "This is not the first backlash reaction we've had to demographic change," he says. "When you have a period of economic uncertainty and inequality combined with rapid demographic change, like the 1850s or early 1900s, [you get] what we are seeing now. But our history of immigration and incorporation is that the backlash doesn't last that long because it becomes politically untenable as the population changes and the [immigrants] become voters."

Yet exactly if and when that tipping point will come remains emphatically unclear. Democrats are benefiting as racial change spreads to more places, but simultaneously Republicans are consolidating their hold on the parts of America least touched by diversity. Those offsetting dynamics have produced a precarious partisan balance -- and a country increasingly separated by a widening demographic divide.