In 2004, Democratic Web sites were convinced that the polls were biased toward George W. Bush, asserting that they showed an implausible gain in the number of voters identifying as Republicans. But in fact, the polls were very near the actual result. Mr. Bush defeated John Kerry by 2.5 percentage points, close to (in fact just slightly better than) the 1- or 2-point lead that he had on average in the final polls. Exit polls that year found an equal number of voters describing themselves as Democrats and Republicans, also close to what the polls had predicted. ...538
Nate Silver searches for "biased" poll results. Republicans, lately, seem to be sure the latest polls, showing a healthy lead for Obama, are biased. Democrats had the same feeling back in 2004. But the bias turns out to be only perception, not actuality.
Silver, after a close look the possibility for bias, has this to say:
... It is reasonably impressive how unbiased the polls have been. In both presidential and Senate races, the bias has been less than a full percentage point over the long run, and it has run in opposite directions.
That does not mean the pollsters will necessarily get this particular election right. Years like 1980 suggest that there are sometimes errors in the polls that are much larger than can be explained through sampling error alone. The probability estimates you see attached to the FiveThirtyEight forecasts are based on how the polls have performed historically in practice, and not how well they claim to do in theory.
But if there is such an error, the historical evidence suggests that it is about equally likely to run in either direction.
Nor is there any suggestion that polls have become more biased toward Democratic candidates over time. Out of the past seven election cycles, the polls had a very slight Republican bias in 2010, and a more noticeable Republican bias in 1998, 2000 and 2006.
They had a Democratic bias only in 2004, and it was very modest.
Still, 2004 went to show that accusations of skewed polling are often rooted in wishful thinking. ...538
Polls can't be expected to come up with fail-safe results. But "less than a full percentage point over the long run" is pretty damn good. And whenever you have doubts, check FiveThirtyEight