Why were the polls (especially national ones) so off?

[tagimaucia]

It isn't really talked about the way the rust belt polling miss in 2016 is, because all the quants pretty much predicted Obama would win and he did, which is a lot less interesting than an upset-- but the national polls were basically ~3 points off in 2012 on average.

Swing state polls tended to be a bit better, but they also tended to underestimate Obama by a point or two on average (I think, someone can fact check me on this if I'm wrong).

Has anyone done a deep dive into why the polls were so off, particularly national ones?  Can it mostly or entirely be explained by pollsters assuming that Obama couldn't duplicate 2008 turnout levels with black/minority voters, which he did?  Are there any other clear reasons?

[tagimaucia]

I'm not a statistician, but I don't think "margin of error" is really a concept to be applied to averages of polls.  It just represents a 95% confidence interval for each individual poll, doesn't it?

Obama won by 3.9% and even if you take away Rasmussen and Gallup, the average of the final polls included in the RCP average was 1.1%, which is a significantly bigger gap between the polls and the actual result than in 2016, 2008, or 2004.