Four of the last five surveys has shown Ms. Nunn with an advantage. This morning, a CNN/ORC poll showed Ms. Nunn ahead by three percentage points; another poll, from The Atlanta Journal-Constitution/SRBI, showed Mr. Perdue ahead by two points. All of the recent polls show her well beneath the 50 percent she needs to avoid a January runoff, but if she leads by two or three points, she would have a real shot to win outright.

The most important variable is the minor party vote. For Ms. Nunn to break 50 percent, she needs to top Mr. Perdue by a margin equal to or exceeding the number of votes won by the Libertarian candidate Amanda Swafford. Polls show Ms. Swafford has been attracting about 4.5 percent of the vote, which would make an outright Nunn victory very challenging.