This is a big part of the reason: http://www.amazon.com/The-Big-Sort-Clustering-Like-Minded/dp/0618689354. When everyone you know thinks that Obama is an Atheist Kenyan Muslim Homosexual Socialist, it's difficult to imagine how there can be so many people out there who think any differently. This is a big part of the reason why so many Democrats and liberals were incredulous and shocked when Bush was reelected in 2004, even though he had been leading in the polls for months (even if not by much).
Excellent point. I call it the "herd mentality". It seems to be strongest in urban areas.
Yeah, I've noticed it in the Birmingham, Dallas, and Salt Lake City metro areas too, actually.
