The suburbs of Cobb County, Ga., boomed during white flight on the promise of isolation from Atlanta. Residents there dating to the 1960s did not want Atlanta problems, or Atlanta transit, or Atlanta people. As a local commissioner once infamously put it, he would stock piranha in the Chattahoochee River that separates Cobb from Atlanta if it were necessary to keep the city out.

The county became a model of the conservative, suburban South, opposed to the kind of federal meddling that integrates schools, or the kind of taxes that fund big infrastructure. And then, this year, after timidly embracing Hillary Clinton in 2016 (she won the area by just two points), Cobb County voted for Joe Biden by 14 percentage points. And Democrats swept the major countywide races.

“It’s been this evolution of Cobb from a white-flight suburb to, now, I went to a Ramadan meal in a gated community in Cobb County that was multiracial,” said Andrea Young, the executive director of the Georgia A.C.L.U., and the daughter of the former Atlanta mayor Andrew Young. “This is the story,” she said, “of Atlanta spilling out into the metro area.”

Around the region, suburban communities that once defined themselves in opposition to Atlanta have increasingly come to resemble it: in demographics, in urban conveniences and challenges, and, finally, in politics. Rather than symbolizing a bulwark against Black political power, these places have become part of a coalition led by Black voters that is large enough to tip statewide races — and that could hand control of the Senate to Democrats next month.

“In Atlanta, they thought they could draw a line, and they thought it would be permanent, whether it was the Chattahoochee River, or Sandy Springs forming its own city to keep Atlanta out,” said Kevin Kruse, a Princeton historian whose book “White Flight” followed the mass migration from Atlanta in the civil rights era. “That was just a holding operation. It couldn’t stop those forces of progress.”

Mr. Kruse says these suburbs gave rise to a “politics of suburban secession.” Their voters prized private spaces over the public good, low taxes over big government, local autonomy over federal intervention. Newt Gingrich, a House member from Cobb County who embodied that agenda, became House Speaker in 1995. And neighboring counties were as reliably red. In 2004, George W. Bush carried Cobb by 25 points. He carried Gwinnett County to the east by 32 points, and Henry County south of Atlanta by 34 points.