Former Georgia gubernatorial candidate Stacey Abrams, and founder of Fair Fight, sent an encouraging tweet to supporters moments after Warnock took the lead over Loeffler in his Senate race with 95% of the expected vote reporting in.

“With new votes joining the tally, we are on a strong path. But even while we wait for more, let’s celebrate the extraordinary organizers, volunteers, canvassers & tireless groups that haven’t stopped going since Nov. Across our state, we roared. A few miles to go...but well done!” Abrams tweeted.

Gabriel Sterling, the state voting system implementation manager, said at an earlier election update that the largest portion of remaining uncounted votes was out of DeKalb County, a heavily Democratic suburban Atlanta area.

Gabriel Sterling, the state voting system implementation manager, said in an evening election update that the largest portion of remaining uncounted votes is from DeKalb County, a heavily Democratic suburban Atlanta area.

"The biggest tranche of votes that exists out there in a single bucket right now still looks to be the advanced voting in DeKalb County," Sterling said.

With 80% of the expected vote reporting in, GOP candidates Perdue and Loeffler are leading their races by small margins.

Sterling said from the press conference in Atlanta that he wasn't aware of any issues that could delay the reporting but added, "It can be like seesaw for the rest of the evening."

Sterling said 64 of the state's 159 counties have completed reporting all votes.

-ABC News' Quinn Scanlan

Georgia Lt. Gov. Geoff Duncan, a Republican, told ABC News Live he's worried that President Donald Trump's baseless allegations of election fraud in the state "absolutely" might have depressed GOP turnout in the runoff races.

"Unfortunately, too many folks in our party over the last two months have been talking about misinformation, election fraud and creating a distraction," Duncan said.

"We've been outworked, out-focused, but my hope is that Kelly Loeffler's and David Purdue's hard work over the last two months is going to get them across the finish line and help us kind of regroup as a party here in this state, and also as a country, and be able to roll back out and get the White House in 2024," he added.

ABC News Chief Congressional Correspondent Mary Bruce stressed the significance of Tuesday's runoffs, not only because they will determine control of the Senate but because they leave President-elect Joe Biden's agenda hanging in the balance.

"It's not just how much he can get done but also the expediency with which he might be able to get things done," Bruce said, outlining big promises in Biden's agenda, from health care to climate change.

Biden has noted in recent days, while stumping for Democratic candidates, that if Democrats are able to win both seats, the $2,000 stimulus checks Democrats have been pushing for could happen immediately. Also, the approval of additional aid to state and local governments and resources to distribute the coronavirus vaccine in a more timely fashion could come quickly.

"Not to say that Joe Biden's agenda would hit a complete brick wall if Republicans continue to maintain control in the Senate because, of course, Joe Biden prides himself on having a great working and personal relationship with Republican leader Mitch McConnell, but it certainly would make Joe Biden's agenda and his political ambitions here much, much, much easier," Bruce said.

Bruce also noted how the expected efforts by Republicans to object to the certification of the Electoral College results on Wednesday could give the public a sense of the "uphill battle" Biden may face if the GOP does maintain control of the Senate.

Former President Barack Obama tweeted to Georgia voters that while state polls close at 7 p.m., they should stay in line if they've arrived at their polling location in time.

"You have the right to vote, no matter how long it takes," Obama tweeted.

Some precincts in the state will be open past 7 p.m. due to court orders.

Obama joined a virtual rally for Democratic candidates Rev. Raphael Warnock and Jon Ossoff earlier this month and has appeared in digital ads for both candidates. On Monday, the former president tweeted "the stakes could not be higher" in promoting the runoff elections.