Why polls, and any argument based on polls, are complete rubbish:
American polling organisations tend to rely on relatively small samples (certainly judged by British standards) for their results, often somewhere between 500 and 700 likely voters, compared to the more usual 1000-2000-plus for British national polls. The recent New York Times poll that gave Obama a 12 per cent lead was based on interviews with just 283 people. For a country the size of the United States, this is the equivalent to stopping a few people at random in the street, or throwing darts at a board.
Read the rest of the article for a depressing explanation of why we still rely on them. The most amazing fact about American political journalism is that most of it is nonsense, it can easily be shown to be nonsense, and no one cares.