Joe Biden’s victory in Georgia was surely the most surprising red state flip in the 2020 presidential election. Exploring the strikingly rapid transformation of Gwinnett County, the state’s largest suburban county, shows how this upset happened, while offering insights into what to expect in the state’s all-important upcoming U.S. Senate runoff races.

At the midpoint of the 20th century, prospects for Gwinnett, situated some 30 miles northeast of Atlanta, seemed little brighter than those of any other outlying county. It was largely rural, with just 32,000 inhabitants, more than 95 percent of them white. Scarcely 2 percent of the residents had completed four years of college, and only 25 percent of the workforce held professional or managerial jobs. Even the most prescient observer could hardly have imagined that today, Gwinnett County would be home to nearly 950,000 people, and one of the most diverse counties in the United States.

Its path would be shaped by nearby Atlanta, which lost 162,000 white residents between 1960 and 1980, on its way to becoming a majority Black city around 1970. Many of the whites fleeing Atlanta resettled in Gwinnett, which saw its white population swell by 102,000 over the same period. As time went on, the county also grew with the arrival of thousands of white migrants from elsewhere in the nation, drawn there by the dynamic metropolitan Atlanta economy. By 1990, some 357,000 people, more than nine out of 10 of them white, lived in Gwinnett County.

Growth in Gwinnett would accelerate, even as the new arrivals became more diverse. By 1990, metro Atlanta, including suburban counties like Gwinnett, was becoming a top destination for African Americans on the move from other parts of the U.S. Meanwhile, in keeping with a national pattern, the allure of the suburbs was accelerating the outmigration of upwardly mobile Black people from Atlanta itself, which eventually spilled over into Gwinnett from closer-in Dekalb County. As a result, the share of Gwinnett’s population that was African American more than doubled in the 1990s, and the Black population grew by 140 percent in the decade that followed. With this influx continuing, and many Gwinnett whites heading off to the decidedly whiter exurbs like Barrow and Walton counties, by 2020, 30 percent of Gwinnett residents were Black.

This is only part of the story of Gwinnett’s diversification. The explosion of construction jobs spawned by the 1996 Olympic Games, in combination with a falloff in employment in the Texas and Louisiana oilfields, exerted a push-pull effect on Latino in-migrants, so that today, Latinos constitute 20 percent of the county’s population. The heated pursuit of foreign investment, reflected in a current count of more than 600 foreign-owned companies in the county, also drew a steadily expanding stream of Asian and Asian American workers, who liked the county’s highly touted schools and relatively low living costs. By 2020, residents of Asian descent accounted for 13 percent of Gwinnett’s population.

These arrivals have made Gwinnett more prosperous and diverse, but they were not universally welcomed by the increasingly outnumbered white population, which stubbornly resisted ceding influence over local affairs. While Gwinnett officially became a majority non-white county by around 2007, it wasn’t until 2018 that non-white representatives first managed to win seats on the county commission and the board of education.

As we know, white resistance to nonwhite political advancement is hardly confined to the South—but here, as elsewhere, it has had partisan implications. Gwinnett became a Republican stronghold in presidential elections in 1964, when its white voters joined a mass exodus of Southern whites from the Democratic Party in response to the civil rights initiatives of the Kennedy and Johnson administrations. Gwinnett’s voters strayed from the GOP camp only once in the next 50 years, to support fellow Georgian Jimmy Carter for president in 1976.

Over time, however, as the county’s nonwhite population grew, Democrats began to gain ground in Gwinnett during presidential elections. Democrats had long been able to count on carrying Black majority Fulton County, During the 1990s they would pick up adjacent Clayton and DeKalb counties, which also diversified and moved into the Democratic column.

Barack Obama turned heads in 2008 by running only five points behind John McCain in Georgia. He captured Newton, Rockdale, and Douglas, three more metro Atlanta counties whose nonwhite populations were on the rise. Obama maintained this suburban beachhead in 2012, and, four years later, continuing growth of suburban minority populations helped Hillary Clinton pick up three more counties, Henry, once-notoriously far-right Cobb County—and Gwinnett, which turned blue for the first time in 40 years.

With Democratic hopes buoyed by Clinton’s showing, state legislator Stacey Abrams, who is African American, ran for governor in 2018 and nearly prevailed, losing to Republican opponent Brian Kemp, who is white, by a margin of less than 1.5 percent statewide—and besting him in Gwinnett County by more than 14 percent. Abrams’s performance persuaded the national Democratic Party that it was finally time to mount a serious presidential effort in Georgia.

This time, however, instead of persisting in their perennially futile efforts to bring blue collar whites back into the fold, they would concentrate on molding a growing and more politically engaged nonwhite population into a potent political force in the state. To that end, Abrams and her allies, including a number of other Black women who are seasoned political activists, came together fight voter suppression, expand registration, and boost turnout. The effort clearly enhanced Biden’s fortunes in Georgia, where he carried essentially the same counties as Abrams two years ago, though he bested her number of votes statewide by more than 550,000.

While the Atlanta suburbs might be seen as gradually “purpling” over the last two decades, it is important to note the neck-snapping speed of this reversal in Gwinnett, which swung 27 points in less than a decade. Obama lost the county by 9 points in 2012 only to see Biden win it by 18 points just eight years later.

Yet the reasons behind that swing bear closer examination. They don’t really square with the early “hot take” that Biden owes his victory nationwide to disgusted white establishment Republicans in the suburbs, who crossed party lines rather than vote for Donald Trump but otherwise stuck with GOP candidates down ballot. Trump certainly lost some support among white suburban voters in Georgia. But ultimately, it was a racially and culturally diverse group of relative newcomers to the ‘burbs who put Democrats over the top in the state.

Sure, the pundits are right that Biden won in the suburbs—the state’s three most populous suburban counties (Dekalb, Cobb, and Gwinnett), along with Fulton, accounted for over half of Biden’s vote gains over Hillary Clinton four years ago. But it’s also true that of those counties, Cobb is the only one where nonwhites are not the majority. (And Cobb misses being majority non-white by a single percentage point.)

There are limits to what may be inferred from comparing voter registration percentages to shares of the actual vote, but in Gwinnett County the correlation is too close to ignore: whites now account for 40 percent of the county’s registered voters, and Trump won 40 percent of the vote. Likewise, in Georgia’s Seventh District, the only race where the Democrats managed to flip a House seat, Carolyn Bourdeaux’s vanquished Republican adversary effectively garnered the same share of the vote in Gwinnett as Trump.

Moreover, the perceived national pattern of white Republicans in the suburbs voting for Biden but holding the line for the GOP in other races certainly did not play out in Gwinnett. Instead, Democrats racked up large margins in the two senatorial contests, while local matchups saw more ousters of white Republicans by non-white candidates, including the election of Gwinnett’s first Black sheriff. Here—and, one suspects in Georgia’s other large, diverse suburban counties as well—Democrats didn’t raid the traditional Republican base, but simply overwhelmed it by turning out their own expanded base in force.

Despite this, Republicans in January’s U.S. Senate runoffs are doubling down on their presidential election strategy of tailoring their message to the die-hard Trumpers, and casting Democratic challengers Jon Ossoff and Raphael Warnock as America-hating socialists, communists, or anarchists out to destroy our way of life.

In this, they seem to be gambling that the traditional wisdom that non-white turnout fades badly in run-off elections will hold true yet again. Yet this wisdom may no longer hold true, especially in the suburbs, which are now so much more diverse than in the past. In fact, the Democrats’ chances in the runoffs may actually be enhanced because one of the Democratic candidates, Warnock, is Black.

Should Georgia Democrats prevail in these two races, they will have managed an even greater feat than the one they pulled off on Nov. 3. They will also have shown the rest of the country that the demographic forces transforming Georgia’s politics could fuel further Democratic success in rapidly diversifying suburban areas nationwide.