I don't believe there will be such a huge drop-off this time, if only because we're not dealing with a landmark election year with the highest black turnout in history and historically unstable electoral performance. The people who vote in mid-terms are much more likely to come back out to vote in a run-off when compared to a presidential electorate. I'd expect a 49-46 Nunn lead in November to become a 53-47 Perdue victory in January (although the fact that there'll be two run-offs - one for Gov and one for Sen - could throw a wrench into that).
The national party also won't completely forego Georgia in a 2014 runoff like they did in 2008 (which in hindsight, turned out to be a really stupid decision, no?).
No. Bill Clinton and Al Gore did campaign for Jim Martin, and didn't do any good. Obama would have wasted political capital by investing in a losing Senate race 3 weeks after becoming President-elect.
