Understanding the polls

There is a massive discrepancy between National polls and State polls.  The National polls put Romney ahead.  The State polls suggest Obama has the edge.  This article by Nate Silver is an excellent summary of the issues involved and this is another that examines the issue in a slightly different way.  

The short summary is that we don't really know why National polls are disagreeing with State polls.  Essentially, if you take a weighted average of state polls it should equal the national polls.  Except, currently that isn't what is happening.  Simple explanations like non competitive state margins don't quite seem to explain it.  It's tempting to argue that it is because of "sampling error", but there would have to be a pretty significant bias in the state polls for all of them to skew so dramatically to Obama.  Nate Silver and others believe that state polls may be more unbiased and their larger samples make them more stable.   Another hypothesis is that things are being messed up because the "likely voter" models are wrong - that there are state by state differences which pollsters are inadequately adjusting for.  However, we won't know for certain until the results, and perhaps not even then.