(AFP) - Hopeful Democrats were scenting victory Tuesday night in two knife-edge runoff races in Georgia that are set to determine the balance of power in the US Senate under Joe Biden's presidency.

The hard-fought races have brought the international spotlight onto this southern state, nine weeks after the most tumultuous American election in two decades.

Democrats need both their candidates, Jon Ossoff and Raphael Warnock, to prevail in order to snatch the Senate from the Republicans. Outgoing President Donald Trump's party needs either Kelly Loeffler or David Perdue to hold their seat to preserve its majority.

With more than three quarters of precincts reporting, the two races were within a percentage point.

Both Republicans were narrowly ahead, but US media reported that many of the outstanding votes were in metropolitan Atlanta, a major Democratic stronghold.

That was enough for Democratic Representative Ilhan Omar to declare: "Georgia will make history again."

"Wow. I think we're gonna win," tweeted Rufus Gifford, a former ambassador who was deputy head of Biden's 2020 campaign.

Georgia officials have said results may not be known until Wednesday or later, but they have also reported the runoffs have been "running smoothly."

"Georgia -- The nation is looking to you to lead us forward," tweeted President-elect Biden earlier on Tuesday.

"The power is in your hands," wrote the 78-year-old Democrat, who like Trump visited the Peach State to rally supporters on the eve of the vote.

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

On Wednesday, Vice President Mike Pence is to preside over a joint session of Congress to certify the Electoral College vote that confirmed Biden as the winner of the White House.

Trump is planning to address a rally near the White House in protest at the certification, with hundreds of supporters in red "Make America Great Again" caps already gathered downtown Tuesday amid a heavy police presence, as shop owners, fearing unrest, boarded up windows.

Trump has urged Pence to "reject fraudulently chosen electors" -- a move the vice president has no legal authority to make -- while dozens of House Republicans and 12 Senate Republicans have separately said they will raise objections to certification, a move doomed to fail.

- 'Continued obstruction' -

Biden beat Trump in Georgia by nearly 12,000 votes in November, but the runoff odds are not in Democrats' favor: Republicans have won all eight statewide runoffs since 1992.

Democrats led the early voting in Georgia and nationwide in November, and the expectation is that they did so again in the runoffs, in which a record three million-plus people voted early amid the coronavirus pandemic.

Voting figures show Republicans more than Democrats tend to prefer casting ballots in person on election day.

Robert Lowe, a 74-year-old retired improv comedian, called it "the most important election in my lifetime," after casting his ballot at an Atlanta church for the Democrats.

If Republicans retain control of the Senate there will be "continued obstruction" in Washington, Lowe told AFP.

- High tensions, high stakes -

Republicans hold 50 seats in the 100-seat Senate and a victory in just one of the runoff races would give them a majority and the ability to thwart Biden's agenda.

Ossoff, 33 and a documentary producer, is running against Perdue, a 71-year-old business executive who was elected to the Senate in 2014.

Warnock, a 51-year-old Black pastor, is taking on Loeffler, 50, a businesswoman appointed to the Senate in December 2019.

Voting figures in several Georgia counties showed Republican slippage against Trump's numbers from November, which could spell bad news for Loeffler and Perdue if that trend continues.

"I've seen enough," tweeted Dave Wasserman, an expert on congressional races and editor of the nonpartisan Cook Political Report, who confidently predicted Warnock would defeat Loeffler -- while cautioning there was "still a ways to go" before the second race was settled.

- 'Find 11,780 votes' -

"It turns out when your coalition depends so much on one person (Donald Trump) driving turnout, there are some big downside risks when that person isn't on the ballot," wrote Wasserman.

Trump's get-out-the-vote rally in Georgia Monday came a day after The Washington Post published a recording of a phone call in which the 74-year-old president pressured Georgia officials to reverse the certified vote of November's election and hand him victory in the state.

On the tape, Trump is heard telling Georgia Secretary of State Brad Raffensperger he wants to "find 11,780 votes" -- one more than Biden's margin of victory.

© Agence France-Presse