One reason for Nelson's edge is his high likeability numbers. According to the poll, 51 percent of Florida voters view him favorably, with 21 percent giving him a very favorable rating. Just 38 percent view him unfavorably, with 20 percent seeing him as very unfavorable.

On the other side of the ticket, only 40 percent of voters give Mack a favorable rating, while 48 percent see him as unfavorable. Just 17 percent see him as very favorable, while 24 percent see him as very unfavorable.

On likeability, Rasmussen said the results from the latest poll are little changed from the mid-August version.

Another key edge for Nelson is his advantage with female voters. The two candidates are neck-and neck when it comes to male support, but Nelson has a 13-point margin when it comes to female voters, 47 percent compared to Mack's 34.

Nelson also has stronger party support, as 82 percent of Florida Democrats back Nelson, compared to 76 percent of Florida Republicans who support Mack. When voters do not identify with a particular political party, Nelson again is in front, 45 percent to 31.

The telephone survey reached 500 likely voters in Florida, and was conducted on Sept. 12.