If neither candidate gets to 50% (there is a third-party candidate), then there will be a runoff on December 2. With 20.2% of the results in, Saxby Chambliss, the incumbent Republican, is holding 59.4% of the vote. Remember, Fulton and DeKalb, particularly heavily Democratic DeKalb, are often last to report.
Chambliss needs to hold 50%. If there is a special election, those races heavily favor the energized. With Bruce Lundsford neck and neck with Mitch McConnell in Kentucky (Arjun Jaikuma is pessimistic about Lundsford's chances based on which counties have come in), Dems may well need this seat to reach 60.
We've already had emails from Obama organizers from around the country who have purchased their tickets to Georgia to win the 60th seat. That's energy. If Chambliss doesn't reach 50, FiveThirtyEight is getting back in the car and heading for Georgia.