Zero.
The Democratic brand is DEAD in the Deep South at the moment. Nunn's chances are highly overstated and Landrieu is looking to be the next Blanche Lincoln. I could see either/both of them somehow winning, but only if the GOP really screws up the next few months. The Democrats will need to look for opportunities to expand elsewhere for the time being. There is a way to win some Southern states without turning the party into moderate heroism but the fundraising base probably wouldn't like the strategy. Namely, get very serious about overturning Citizens United and taking a stronger stance on economic populism. The South has the lowest voter turnout in the nation and if people get the idea that the Democrats are actually serious about Social Justice they could turn out in high numbers. I mean, all those bad stats you guys list? You think all of those people are Republicans? Chances are, given how polarized the South is by measures like class and race, more than a few of those people are Democrats who don't bother voting under the impression that the vote means nothing. Ds need to work on the impression that it does mean something, which I believe is where their strategy is failing. You can't count on Republicans being dumb forever guys.
No Landrieu is not gonna lose as bad as Blanche Lincoln did in 2010. Landrieu may lose but not in Blanche Lincoln 2010 or Rick Santorum 2006 style.