People are influenced by all sorts of considerations and poll results are often extraordinarily sensitive to a myriad of factors. One way in which they may be influenced occurs when their candidate is way ahead in the polls. They may stay home. They can easily be rephrased tendentiously. Now it seems like a large difference.
None are foolproof, however.
I believe that sampling methods should be used. The topic of my earlier ABC column, A Tale of Two Numbers, dealt with the issue. As I said there, the Bureau of Standards wouldn't use the length of Benjamin Franklin's forearm as a unit of measurement and the Census Bureau should not be hobbled by analogous constraints. It depends on the variability of the population and the statistic being calculated. Often the width of the interval is inversely proportional to the square root of the number of people sampled.