Joe Biden’s narrow lead in Georgia and the runoffs that will determine control of the Senate have raised questions as to whether 2020 is a Trumpian aberration or a long-anticipated political shift.

Supporters of Joseph R. Biden Jr. at a rally in Macon, Ga., in October.
Supporters of Joseph R. Biden Jr. at a rally in Macon, Ga., in October.

MARIETTA, Ga. — It took a lifetime for Angie Jones to become a Democrat.

As a young woman, she was the proud daughter of a conservative family active in Republican Party politics. Ten years ago, after a friend’s son came out as gay, Ms. Jones became an independent, though one who still watched Fox News. After the 2016 election, Ms. Jones, a stay-at-home mother in Johns Creek, a pristine wealthy suburb north of downtown Atlanta, became frustrated with seeing her conservative friends defend President Trump through scandal after scandal.

And this year, she voted for Joseph R. Biden Jr., after spending months phone banking, canvassing and organizing for Democratic candidates with a group of suburban women across Atlanta.

“I feel like the Republican Party left me,” said Ms. Jones, 54. “It very much created an existential crisis for me.”

This week, the political evolution of voters like Ms. Jones drove Georgia to a breakthrough for Democrats: Mr. Biden, the president-elect, is on the verge of adding the state to his winning electoral margin, with a narrow lead that is headed for a recount but is nevertheless a dramatic sign of the shifting politics of the South.

And two Democratic candidates, the Rev. Raphael Warnock and Jon Ossoff, forced runoff elections on Jan. 5 that are expected to decide control of the Senate, and thus the fate of Mr. Biden’s agenda. With the November election barely over, the nation’s political focus will now turn to Georgia as much as the presidential transition in Washington, as both parties pour money and resources into what may be an epic, double-feature political battle, in a state that was considered safe Republican territory just a decade ago.

“It’s going to be hand to hand, street by street, and either one of them can win,” said former Gov. Roy Barnes, the most recent Democratic governor in the state. “If Georgians think they’ve gotten sick of political ads, they haven’t seen nothing yet.”

The runoff contests will take place in a Georgia that bears little political resemblance to the state that voted out Mr. Barnes in 2002 after he engaged in a contentious effort to remove the Confederate battle emblem from the state flag.

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The population surged to 10.6 million in 2019 from 7.9 million in 2000, and the number of foreign-born people now exceeds 10 percent of the state’s total population. Atlanta has gone from corporate bland to youthful dynamic, with its indoor food markets, destination dining scene and rich entertainment culture, including a hip-hop scene that drives trends worldwide. The electorate is changing, too: In 2004, 70 percent of voters were white, according to exit polls. This year, white voters made up 60 percent.

Until the 1970s, Georgia was virtually a one-party state, with conservative Democrats dominant. But as conservative voters moved en masse to the Republican Party, Democrats were left concentrated in places like the city of Atlanta, adjacent and urbane Decatur, and smaller cities with significant African-American populations.

In the last few years, however, distaste for Trumpism has spread and demographic change has exploded, giving Democrats surprising new strength in the populous, vote-rich northern suburbs of Atlanta — places like Cobb and Gwinnett Counties that were once bastions of Republican power.

Hillary Clinton carried Cobb and Gwinnett Counties even as she lost the state in 2016. This year, Mr. Biden won them again. And he added to her margin in the state, significantly increasing the number of voters who voted Democratic in the counties that are home to Georgia’s most important second-tier cities — Augusta, Columbus, Macon, Savannah and Athens.

In recent months, the pandemic has slowed the Georgia economy, with unemployment rising to 6.4 percent in September from 5.7 percent in August. Covid-19 cases are increasing too; over the past week, there have been an average of 2,242 new cases per day, an increase of 42 percent from the average two weeks earlier.

A new generation of Democratic candidates has left behind the fiscal and social conservatism of its forefathers to embrace a rising demographic coalition of Black voters, college-educated suburban women and a more politically engaged younger generation.

Even if Mr. Biden’s victory was ultimately delivered by Northern industrial states like Pennsylvania, his slim margin in Georgia most likely points to the future of the Democratic Party, a future which come into clearer view in the Senate runoffs.

Republicans will try to stop any Democratic momentum, hoping to show that Mr. Biden’s strong performance here is more about Mr. Trump’s divisive candidacy than the G.O.P.’s loosening grip on the dynamic South. For Democrats, winning both seats would leave the parties at a 50-50 split in the Senate, with the vice president, Kamala Harris, casting the tiebreaking vote.

Even one Senate seat could be proof that the demographic changes that Democrats have long predicted would favor them in key swaths of the Sun Belt have finally arrived.

And the lessons learned through the organizing of Black and young voters and the energizing of the suburbs are being watched closely by Democrats in other Sun Belt battlegrounds like Texas and Deep South states like South Carolina and Mississippi, where the idea of truly competing with dominant Republicans remains a distant dream.

When Ms. Harris campaigned last weekend in Gwinnett County, now the heart of an emerging, more diverse Georgia, supporters waved signs in Chinese and Spanish.

As a lively crowd waited for Ms. Harris in the vast parking lot of a multipurpose arena, Munir Meghjani, 32, a commercial real estate agent and a practicing Muslim, said that the vice-presidential candidate’s multicultural roots resonated in the changing county.

“I see more and more people like me. I see more and more people of color. I see diversity increasing every single day, not only in Gwinnett County, not only in Atlanta, but across Georgia,” he said. “We’ve always been here, but we’ve always just stayed at home.”

Xiang Li, 36, who works for an insurance company and lives in the county, voted for Mr. Trump in 2016 but said he couldn’t do so again this year.

“I’m voting Democrat in the runoff because of what McConnell has done to the Senate,” Mr. Li, who is registered as an independent, said in reference to the Senate majority leader, Mitch McConnell. “Next election after that, I’ll see what’s going on. I’m hoping the Republican Party comes back to normalcy.”

Despite the changes in Georgia, the Republican Party remains well-organized, powerful and popular in the state, maintaining a stronghold in rural and exurban communities. Democrats had hoped to gain control of the State House to pressure the Republican governor, Brian Kemp, to expand Medicaid under the Affordable Care Act, enact changes to the voting system and give them some voice in the redrawing of congressional and state legislative maps after the 2020 census.

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But they are likely to pick up only a couple of seats once the final votes have been tabulated; the State Senate also remains firmly Republican. The Republicans were also able to pick off the House’s Democratic minority leader, Bob Trammell, an embarrassing loss for the Democratic Party. And redistricting could cost Democrats some of the gains they have made over the past four years.

“If we get a President-elect Biden — and I’m not saying we will — Democrats will be fat and happy, and Republicans will be scared and mobilized, and maybe even angry,” said Brian Robinson, a Republican political strategist in Georgia who predicted twin victories in the Senate runoffs. “And they’re going to want to build a red wall in the Senate.”

The intense political spotlight on Georgia is not entirely unexpected. In 2012, President Barack Obama’s strategists weighed whether to compete in Georgia before deciding that other states provided an easier path to the White House. Four years later, Mrs. Clinton lost the state but won more raw votes than any Democratic nominee in history, a sign that some suburban revulsion for Mr. Trump had already begun.

Opposition to Mr. Trump accelerated the political changes in 2018, helping Democratic candidates flip a House seat and nearly win another in Atlanta’s suburbs. That seat flipped Democratic this month, so far the only straight Democratic pickup in the country. (Democrats also won two districts in North Carolina that had been redrawn to favor them.)

In the meantime, the Democratic nominee Stacey Abrams lost the closest race for governor in a quarter century. Rather than try to woo more conservative white voters, Ms. Abrams focused on turning out intermittent and Black voters while embracing an unapologetically liberal platform.

Her nonprofit group, the New Georgia Project, has registered hundreds of thousands of new voters and served as an inspiration to other emerging activists in the state.

Julius Thomas, 24, a risk analyst broker who grew up in the small coastal city of St. Marys, Ga., moved to Atlanta in 2018, the year in which Ms. Abrams narrowly lost the governor’s race but gained national stature.

Mr. Thomas, who is Black, helped found a nonprofit civic group called the People’s Uprising. They have held voter registration drives targeting young people of color — a recent Juneteenth voter drive featured Atlanta rap stars like Lil Baby — signing up new voters at Black Lives Matter marches and among high school students on campus.

“There’s a culture shift that’s happening among young Black people where it’s no longer cool to sit out civically and not know who your local officials are,” he said. “On your Twitter feed, you’re seeing people say, ‘If you haven’t voted, what’s wrong with you?’”

In the north Atlanta suburbs, Laurie Jules, 39, also Black, said she sees the change.

“In high school, voting was something my parents did,” said Ms. Jules, who is studying for a degree in business administration. “I have a teenager in high school who is a junior, and her and her friends talk about policies, politics. They go to rallies.”

Some of the changes in Georgia mirror the national divides, with urban areas growing increasingly Democratic, rural regions moving toward Republicans and the suburbs left as the country’s most fiercely contested political battlegrounds.

In Georgia’s suburbs, college-educated women have become a powerful force in Democratic politics, forming volunteer networks and organizing for local and national candidates.

Taking a break from putting up some early Christmas decorations, Debbie O’Dekirk and Kassie Jones, two old friends in the affluent northwest Atlanta suburbs, where neighborhoods are dotted with farmer’s markets and lush town squares, described how they had turned against Mr. Trump over the past four years.

Ms. O’Dekirk, 67, initially liked that the president wasn’t a typical politician, voting for him in 2016. But she expected him to take guidance from more experienced Republican leaders once he entered office.

“The exact opposite happened,” said Ms. O’Dekirk, a retired retail executive from Cobb County, who voted for Mr. Biden this year. “It was the biggest disappointment I’ve ever had in my life, as far as any president.”

Ms. Jones, 68, supported Mrs. Clinton four years ago and was devastated when she lost. Since then, she has become increasingly vocal about her support for Democratic candidates — a trend she sees reflected in her politically divided suburban neighborhood. Her street is now dotted with signs for both Mr. Biden and Mr. Trump, and some neighbors have formed a liberal political volunteer group.

“When I first came to Georgia, it was just this private little society of Democrats,” said Ms. Jones, a former banker who moved to the state shortly before Mr. Trump was elected. “But the more concerned they got with the president, the more people started to speak up.”

Ms. Jones, who volunteered for Mr. Biden’s campaign, has already starting talking to her neighbors about how they can help Democrats in the runoff elections.

“I’m so proud to be part of Georgia now,” she said. “But we’ve got to get everyone to understand their job’s not done yet.”

Isabella Grullón Paz contributed reporting from New York."