A voter checks in at the Neighborhood Church polling station in Candler Park, Atlanta, Georgia, during the Georgia Senate runoff elections on Monday. Photo: AFP

Joe Biden's Democratic Party took a giant step Wednesday toward seizing control of the US Senate as they won the first of two Georgia runoffs, hours before Congress was set to certify the president-elect's victory over Donald Trump.

Reverend Raphael Warnock's victory, projected by multiple US networks overnight, capped a grueling nine-week runoff campaign and puts Georgia's other knife-edge race in the spotlight for its potential to impact the balance of power in Washington.

"I promise you this tonight: I am going to the Senate to work for all of Georgia," Warnock said in a livestream to supporters.

Should Republicans lose the second race it would be a political debacle just hours before Trump is expected to suffer another bitter blow when Congress affirms Biden's Electoral College victory.

Multiple Republicans have signaled they will try to block the certification of the vote. 

But their numbers appear slim and the House and Senate are all but guaranteed to reject the effort and certify the Electoral College results.

Warnock, 51, made history as just the third African American to win a Senate seat from the South. He defeated Kelly Loeffler, a 50-year-old businesswoman appointed to the Senate in December 2019.

In the second race, Democrat Jon Ossoff's campaign manager said in a statement that once all votes are counted they "fully expect" Ossoff will have defeated his Republican opponent.

With 97 percent of precincts reporting, Ossoff edged out to a 0.22 percent lead, and analysts have said he was well placed to win, with outstanding votes coming from Democratic-­leaning regions such as Atlanta.

Such results would be a major political upset in a GOP bastion, a southern state that has been reliably Republican for two decades but which Biden won in an upset on November 3 as he marched to victory against Trump in the presidential race. 

It would also send reverberations through Washington and the nation, as it would essentially hand Biden's Democrats the levers of power in the executive branch and both chambers of Congress.

Democrats need both candidates to prevail in order to snatch the Senate from the Republicans. If Perdue holds on to win, Republicans preserve their Senate majority.

Georgia was voting during a week of high political tensions, with Trump desperately scheming to reverse his election loss.