Democrat Raphael Warnock has unseated Kelly Loeffler in the Georgia Senate election, NBC news projects.
The final votes are being counted in two key Senate run-off elections that will determine control of the US Senate and the fate of Joe Biden's presidency.
Mr Warnock and fellow Democrat Jon Ossoff need to win both races to seize the Senate majority - and control of the new Congress when president-elect Mr Biden takes over.
A pastor who spent the past 15 years leading the Atlanta church where Martin Luther King Jr preached, Mr Warnock is projected to defeat Republican incumbent Kelly Loeffler by NBC news.
The 51-year-old will become the first black senator in Georgia's history and his victory will be a symbol of a striking shift in the state's politics.
Black voters made up approximately 30% of the electorate and almost all of them - 94% - backed Mr Ossoff and Mr Warnock, according to a survey of 3,792 voters.
Voters under 45 and those earning less than $50,000 (£36,000) also voted for the Democrats, as well as around 60% of recent arrivals - the force behind Atlanta's sprawling growth, the AP VoteCast found.
It follows Mr Biden's victory in November, when he became the first Democratic presidential candidate to carry Georgia since 1992.
Mr Warnock's win will also be a stinging rebuke of outgoing President Donald Trump, who made one of his final trips in office to Georgia to rally his loyal base behind Ms Loeffler and fellow Republican David Perdue.
The focus is now shifting to the second race between Mr Perdue and Mr Ossoff - with it still too early to call as votes are still being counted.
Mr Ossoff would be the Senate's youngest member at just 33, and the Democrats would have control of Congress, ultimately strengthening incoming president Mr Biden's standing as he prepares to take office in two weeks.
In a message to his supporters on Wednesday, Mr Warnock referenced his family's experience with poverty, saying his mother used to pick "somebody else's cotton" as a teenager.
"The other day, because this is America, the 82-year-old hands that used to pick somebody else's cotton picked her youngest son to be a United States senator," he said. "Tonight, we proved with hope, hard work and the people by our side, anything is possible."
His edge is likely to grow as more votes are counted, most of which are in Democratic-leaning areas.
Meanwhile, former businesswoman Ms Loeffler, 50 - who was appointed to the Senate less than a year ago - refused to concede in a brief message to supporters.
She said: "We've got some work to do here. This is a game of inches. We're going to win this election."
