Would a runoff increase or reduce Michelle Nunn's chances?

[Never]

Reduce.

Democratic voters are much less reliable runoff-voters than Republicans.

[Never]

What would be the argument that it could increase her chances? That doesn't make a whole lot of sense to me.

I cannot think of one. She would have to lead in the general election by ten percent in my opinion to win the runoff.

Right, and if she was leading by 10% in the GE there would be no runoff-- that would require a third candidate to pull at least 12% of the vote.

Precisely. If Nunn is somehow leading by ten, she won't go to a runoff. That kind of margin means that something very, very bad happened to damage Perdue's chances.