Here's a handy chart of historical Presidential polling:
http://www.ncpp.org/1936-2000.htmAnd here's my brief analysis of 1992 and earlier that I posted in another thread:
Using that chart, it appears a similar bias has appeared in many other previous Presidential elections as well:
In 1992, the six polls listed predicted a Clinton win of 6-12 points. He won by 6. Again, at the extreme end of the range.
In 1988, the five polls listed predicted a Bush win of 4-12 points. He won by 7. No clear bias, though 4 of the 5 overestimated the size of Bush's victory.
In 1984, the six polls listed showed a huge range: anywhere from 10 point Reagan win to 27 point. Again, no clear bias, but that range is much larger than the statistical error.
In 1980, the four polls listed all predicted a smaller Reagan victory than he got: from 1 to 6 points, when he won by 10. Not a great performance.
Before that, there are very few polls listed for each election, and it becomes very hard to detect any bias if there was one. No obvious one shows up. (Except for 1948 and 1952 when Gallup was 10 points off each time.)
So the 1992, 1996, and 2000 elections show this bias: where the final outcome was only correctly predicted by an "outlying" poll.
Before that, the track record is somewhat better (except for 1980), but that was a very different polling universe than exists today.
So I have every reason to believe that this bias still exists today.
But I also have every reason to believe that, indeed, this is a close race.