If it gets to a runoff (which I think it will), it will depend on:
1) how far Nunn will run ahead of Perdue on November 4 (if at all). She would probably need at least a 2 point lead on Election Day.
2) the Senate breakdown after November 4. If the Republicans have already won the Senate in November, the enthusiasm level goes down (like it did for the Democrats in 1992 Coverdell-Fowler and 2008 Chambliss-Martin), and the runoff will probably favor Nunn
At the end of the day though, the libertarian vote won't be six-percent on Election Day, so a 2-point lead would probably get you very close to 50%. Libertarians will poll around 6-percent in Georgia, but end up in the 2-3 point range when people pull the lever on Election Day.
Don't forget there's also the Governor's race in Georgia. If that goes to a runoff as well, you'll have higher turnout. It will basically be a general election redux.
There would be two seperate runoffs in Georgia. The Governor's race in December, the Senate in early Jan
