A surrogate for the gun control group Everytown for Gun Safety, McBath said pushing for new gun control policies will be her "top priority" upon arriving in Washington.
"We've sent a strong message to the entire country. Absolutely nothing - no politician & no special interest - is more powerful than a mother on a mission," tweeted McBath, a former flight attendant who became a gun control advocate after her teenage son Jordan Davis was fatally shot following a racially-charged dispute in 2012.
Once represented by Newt Gingrich, Johnny Isakson and Price, the 6th District was until recently considered safely Republican. Mitt Romney carried the district by 24 percentage points in 2012 but Donald Trump won it by less than 2 percentage points in 2016.
Handel's ouster comes after Republicans struggled on election night in many suburban seats that were once friendly to the party, including in states like Texas and Kansas. Disdain for Trump, paired with a groundswell of energy on the left, helped turn the tide.
McBath also benefited from elevated turnout from the Georgia governor’s race.
Another suburban Atlanta House seat in the neighboring 7th District was still too close to call on Thursday morning as incumbent Republican Rob Woodall clung to a narrow lead over Democrat Carolyn Bourdeaux.
McBath and Bourdeaux both tapped into national fundraising networks to raise millions, and they received significant assists from liberal mega-donor Michael Bloomberg on Atlanta television, which helped raise their profiles.
Outside political groups largely stayed out of the Atlanta contests as they focused on other races, giving the Democrats room to define themselves on their own terms. And Washington Republicans did not step in to assist Handel in a major way until two weeks ago.
Handel’s campaign strategy was dual-tracked.
In her television advertisements and social media, she largely focused on selling her party's policy victories, including the GOP tax cuts, opioid legislation and anti-human trafficking bills. But on the stump she assailed McBath for her personal finances, brief stint living in Tennessee and connections to Bloomberg, Hillary Clinton and Nancy Pelosi.
McBath, meanwhile, hit Handel for being a Trump sycophant and played up her own personal story. She would start many of her stump speeches by saying she was "still always going to be Jordan's mom."
The Democrat also campaigned heavily on supporting Obamacare and expanding Medicaid.
Handel, who previously served as secretary of state and Fulton County Commission chairwoman, thanked her supporters in a statement on Thursday.
“When I was elected last year, I promised to give you my all in representing you and the entire District. I've done my absolute best to do that – every moment of every day,” she said.
McBath will replace Handel as the only female lawmaker in Georgia’s 16-member congressional delegation. Her seat will become an instant target for Republicans seeking revenge in 2020.