The Democratic Party of US President-elect Joe Biden has won control of the Senate - and of Congress overall with two victories in the state of Georgia.
Raphael Warnock and Jon Ossoff defeated Republican incumbents Kelly Loeffler and David Perdue respectively.
Democrats will control the Senate, the House of Representatives and the White House for the first time since 2009.
An estimated four million Georgians turned out to vote in the run-off election, reports BBC.
The result is a severe blow for outgoing Republican President Donald Trump.
The Georgia election was rerun because none of the candidates in the November general election achieved the 50% needed for victory under state rules.
Mr Warnock, a Baptist pastor, becomes the first black senator for Georgia - a slavery state in the US Civil War - and only the 11th black member of the Senate in US history.
Mr Warnock paid tribute to his mother, Verlene, who as a teenager worked as a farm labourer.
"The other day - because this is America - the 82-year-old hands that used to pick somebody else's cotton went to the polls and picked her youngest son to be a United States senator," he said.
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Raphael Warnock, 51, is a pastor at the Ebenezer Baptist Church in Atlanta - the church where civil rights leader Martin Luther King once preached.
Mr Warnock was a youth pastor in New York City in the 1990s before moving to Baltimore, one of the poorest cities in the US.
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There, he worked to educate his congregation about the risks of HIV/Aids. He also supports access to abortion, making him a rarity among faith leaders, and opposes the death penalty. In 2013, he gave the benediction at the second inauguration of President Barack Obama.
Jon Ossoff, 33, is a documentary filmmaker, born in Atlanta. As a high school student, he interned for John Lewis, the civil rights campaigner and congressman who died in 2020.
He became an aide to a Democratic member of Congress, specialising in foreign policy and national security.
In 2012, Mr Ossoff joined an independent production company specialising in investigative journalism, Insight TWI, which is based in London. He is now the CEO.