The dynamics of a potential runoff are a bit unclear but probably somewhat unfavorable to Nunn:
Georgia’s Senate race went to a runoff in 2008, and Democratic candidate Jim Martin performed drastically worse than he did on the November ballot. But 2008 was a presidential year. There might not be such a disproportionate drop in Democratic turnout in an off-year election.
The runoff, if the outcome of the Senate was still undecided, could turn into a referendum on party control. Voters might place more emphasis on the party identification of the candidates and less on the qualities of the individual candidates. That probably helps Perdue because Georgia is Republican-leaning (although becoming less so).
A third-party candidate, Libertarian Amanda Swafford, would not appear on the runoff ballot. As a group, voters who hold libertarian positions are more Republican-leaning than Democratic-leaning, which could help Perdue. However, Swafford’s positions are split pretty much down the middle between the major parties. Although fiscally conservative, she supports same-sex marriage, marijuana legalization and a reduced U.S. military presence. She’s more of a Gary Johnson Libertarian than a Rand Paul type.
So, there’s a good deal of uncertainty. But the FiveThirtyEight forecast now has Nunn with a 40 percent chance of winning — just slightly worse than a coin flip.
Technically speaking, that projection doesn’t distinguish the Nov. 4 ballot from the runoff. If pollsters were testing a two-way matchup without Stafford on the ballot and trying to evaluate how turnout might differ in a runoff, we might do something more sophisticated, like running separate simulations of the Nov 4. and runoff ballots. Unfortunately, almost none of the pollsters are doing that, so 40 percent has to serve as our stand-in for Nunn’s overall chances of winning the Senate seat.