What is relevant is not the effect on all voters. It is the opinions of potential swing voters whose minds might be changed by evidence. In a close election like this you don't need to care about most potential swing voters - just enough to swing the vote.
This kind of thing is, unfortunately, harder to tell. A simple number certainly doesn't tell it to you.
Another thing that polling is unable to address is how people are affected by information in ways that they think they won't be. You may not care about issue X, but if information on issue X is repeatedly presented indicating that a candidate is lying, you may well be affected. Even though you thought you wouldn't care about issue X.
As evidence that these effects can be significant, on [link|http://www.electoral-vote.com/aug/aug27.html|http://www.electoral...om/aug/aug27.html] the effect of the Swift Boat ads was very noticable.
Cheers,
Ben