"'Independent' voters almost all vote Democrat."
Rush Limbaugh (paraphrased)
His point was, if you haven't made up your own mind this late in an election cycle, you aren't paying attention or thinking for yourself, so you will vote the way the media tells you to, and they all will push the Democrat candidate.
Correction: I think he actually said "Undecided" voters, which is a little different, but not THAT much. You used "fence sitters" and "independent" synonymously, and I went with that. But really, fence-sitters are really the "undecided", and independents technically may not be undecided at this point, so not fence-sitters, they just don't vote the same party every time. However the true "fence-sitters" will probably all identify themselves as independent, when they really aren't. It's a subtle, but important distinction. In any case, I think it's wise to say that you can expect most of those who haven't decided who they are going to vote for late in the cycle are going to vote Democrat if they vote (or someone votes for them).