Finally a name, I am taking a class in cognitive psychology and finally we have encountered the classic problem with much scientific research, especially opinion polls.
I would postulate that it is virtually impossible to do almost any "pure" research that does not have confirmation bias.
Confirmation Bias is defined as "the tendency to selectively look for information that conforms to our hypothesis and to overlook information that argues against it"(Goldstein, Bruce, Cognitive Psychology, 2nd Ed. 2008, Thompson Wadsworth. Now many scientists think that if they go into research assuming nothing that they have no bias. But I would say that by assuming that there is no mover of events or maker of matter they are using their own Bias. Likewise persons of faith assume that there is a creator will use that assumption behind their research.
Of course opinion pollsters are subject to their in the way that they word their questions or just the tone of voice in which they are couched. I have taught my new voters(a 21 year old and an 18 year old) to be intelligent voters and to read between the lines. Always ask what were the questions asked in the poll, what were you actually trying to measure, support for your candidate or the mood of the nation, and how did you go about measuring it?