I don't think Nunn can win statewide without carrying Cobb and Gwinnett.
She could pull it off, but both counties would need to be
close. Nunn's strength is going to come from whittling away at the Republican totals statewide and hopefully cobbling enough together to hit 50% + 1 (because GA has run-offs and she loses in a run-off).
Nunn would have to flip at least a handful of counties in southeast GA. I don't see north GA swinging Dem any time soon.
She will; I expect there to be some substantial swings in central and southern Georgia. The only problem is that outside of Muscogee, Bibb, Chatham and Richmond, this half of the state is pretty desolate (~20% of the population). Compared to 2010, I would expect her to be able to shrink the margins to 2008/Martin levels in many north Georgia counties (she's not black, after all), and that frankly is enough to put her in contention if she pulls it together throughout the remainder of the state. She has a shot of breaking 40% in the sparsely remaining old Democratic bastions in the north like Chattooga; I'll be looking at counties like that as soon as the polls close on Election Night to see whether or not rural support is where it needs to be in order to win.
Here's a uniform swing, winning scenario for Nunn that I did about a month ago: