Georgia Republican Rep. Karen Handel has conceded to Democratic challenger Lucy McBath in a highly watched contest in the state's Sixth Congressional District.

The win is a major upset win for Democrats in a solidly conservative district that's been held by Republicans since Newt Gingrich was first elected to Congress in 1979. Handel won the seat last year in a special election after Tom Price left Congress to join the Trump administration as secretary of Health and Human Services.

"After carefully reviewing all of the election results, it is clear that I came up a bit short on Tuesday," Handel wrote on Twitter.

McBath, a former flight attendant turned gun-control spokeswoman, decided to run for the seat after her son, Jordan Davis, was killed at a Florida gas station in 2012 by a white man who said he shot at the car Davis was sitting in after arguing with the teens over loud music.

"I'm sure you'll continue to see more parents like myself who are losing their children standing up," McBath, a first-time candidate, told ABC News. "It's just going to happen."

Elsewhere in Georgia, the explosive gubernatorial race between Republican Brian Kemp and Democrat Stacey Abrams remains undecided. Kemp currently leads by nearly 63,000 votes with 100 percent of precincts reporting, but Abrams has refused to concede.

"Our opponent has had his office declare himself the victor and we do not accept that," the Abrams campaign told reporters Wednesday night. "They are trying to force an outcome … without proof … and expecting everyone to go along with it,"

Abrams hopes that once provisional and overseas ballots are counted, she will gain enough votes to push Kemp below the 50-percent threshold, which would trigger a run-off election. But experts say Kemp's lead is large enough that that outcome appears unlikely.

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There are two high-profile Senate races that also remain undecided as of Thursday morning.

In Arizona, Republican Rep. Martha McSally narrowly leads Democrat Rep. Kyrsten Sinema by about 17,000 votes. But the Arizona Republic estimates that nearly 650,000 votes remain uncounted statewide, including upward of 600,000 in two counties that traditionally lean Democratic. The latest update on the vote count is expected tonight at 5 p.m. local time (7 p.m. Eastern).

In Florida, Democratic Sen. Bill Nelson has closed the gap with Republican Gov. Rick Scott, and the two are separated by less than half of a percentage point, which would trigger an automatic recount.

Democrats have now successfully flipped 29 seats in the House, winning 223 compared to the 197 seats won by Republicans, according to the Associated Press. Fifteen House races remain uncalled, and several of those are in California, which is traditionally slow to count mail ballots.

Closer to home, Democratic challenger Andy Kim has claimed victory in New Jersey's Third Congressional District, but GOP incumbent Rep. Tom MacArthur has not conceded.