Why have they underpolled Obama and overpolled Clinton so badly this cycle?
Remember their last Iowa poll?
Clinton-34% Edwards-21% Obama-25%
They missed Obama by 13 pts, how does that even happen?
Because they're really awful. I'm convinced that they just use a random number generator to create their "polls". They've regularly been off by 10 points or so, and 15 points off isn't that uncommon. In SC, their final pre-election poll had Obama leading by 3, and he won by 24.
Here are some of ARG's greatest hits just from pre-Super Tuesday (final ARG poll for each state):
IA: Clinton up by 9
NH: Obama up by 9
MI: McCain up by 1
SC: Obama up by 3
SC: Huckabee up by 7
FL: Romney up by 2
But those are just the final pre-election polls. ARG's polls from weeks or months before an election are often much worse. For example, in early 2007, they had Clinton leading by double digits in Iowa, when every other pollster had Edwards leading. In the spring/summer of 2007, they had McCain still leading in SC, when everyone else had him trailing by double digits. In mid-January, when every other pollster had McCain leading in CA and Giuliani in the low teens, ARG had Giuliani leading by double digits with 33% of the vote.