Nate Silver finds no consistent bias in the history of presidential polling:
In the 10 presidential elections since 1972, there have been five years (1976, 1980, 1992, 1996 and 2004) in which the national presidential polls overestimated the standing of the Democratic candidate. However, there were also four years (1972, 1984, 1988 and 2000) in which they overestimated the standing of the Republican. Finally, there was 2008, when the average of likely voter polls showed Mr. Obama winning by 7.3 percentage points, his exact margin of victory over John McCain, to the decimal place.
In all but three years, the partisan bias in the polls was small, with the polling average coming within 1.5 percentage points of the actual result. (I use the term “bias” in a statistical sense, meaning simply that the results tended to miss toward one direction.)
On the whole, 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.
So in 2012, as Krugman puts it, "the facts have a well-known liberal bias."