Jay Cost doesn’t see malevolent intent, just a history of oversampling Democrats consistently across a variety of polling organizations:

The clear answer is: they skew Democratic. In fact, every recent registered voter poll with party spreads I could identify had a Democratic advantage that exceeded the quarter-century trend. And the average of all these polls together is 35 percent Democratic to 29.5 percent Republican, or D +5.5

Importantly, these polls show Obama with an average lead over Mitt Romney of 3.5 points. But if they have a 2.5-point Democratic oversample, then what we are really talking about is perhaps a 1-point Obama lead.

Incidentally, this puts these polls much more in line with the Rasmussen poll, which has consistently found a toss-up race. Right now, Rasmussen – a poll of likely voters – sees an R+1.4 advantage in party identification. That is entirely defensible, in my opinion, given the weakness in the economy….

Here’s my bottom line. It is very difficult to model the turnout for a presidential election this far away from November. There are a lot of tough choices that pollsters must make, and it is not fair to single any pollster out for the decisions it ultimately goes with. Nevertheless, we can and should still be smart consumers of political polling. We need to keep the historical spread between the two sides in mind, and be cautious of polls that show a relatively wide Democratic advantage over the GOP. They are probably underestimating the GOP’s electoral strength.

I’ve never seen anyone analyze the “hang up” effect, namely, that Republicans (particularly conservatives) hang up on pollsters more than Democrats (particularly liberals).  If anyone has a link to a study of this phenomenon, even if debunking it, please post in the comments.

In the meantime, don’t get overly pessimistic about the polls.  We have enough problems with Operation Demoralize without getting apoplectic.

Update:  Reader Charles forwards this analysis by Nate Silver:

Over the past six presidential election cycles, likely voter polls have been 0.7 points to 2.5 points more favorable to the Republican candidate than registered voter polls….

For the time being, our model is giving Mr. Romney credit for a 1.6-point shift when it sees a registered voter poll and has him just a tiny bit behind Mr. Obama despite this. Needless to say, if the gap comes in toward the higher end of the historical range — or at something higher still, like a three-point difference in his favor — Mr. Romney’s chances of winning the election will improve.