There's still a lot of debate about what went wrong with the polling in New Hampshire. Personally? With the benefit of hindsight, I think that it's clearly, unequivocally the Bradley Effect at work.
Let's cover the bases on why I think that's so.
First of all, the polling on the Republican side was solid and did a good job of reflecting the actual results. So, there was obviously a factor on the Democratic side that was not in play with the Republicans.
Additionally, the polling definitely pointed towards a Barack Obama victory. There were 22 polls done in the last 3 days before the election, 20 of which had Obama winning, along with 1 tie, and 1 small Clinton victory. Moreover, there were 4 polls that actually included data from the last day before the election and Obama won them by 5, 7, 9, and 13 points respectively. If you compare those polls to the polls that included data from the two previous days only, Barack's margin of victory actually appeared to be getting LARGER. (Average victory of 8.5% on the 4 polls that included Monday data, and a 7% average victory on the 5 polls that included only Saturday and Sunday data).
So, did some event like Hillary crying or those radio stunt people holding up signs that said "Iron my shirt" cause a huge shift towards Hillary at the last minute? No, because it would have been reflected in the polling if that were the case.
Could it be that there were nefarious people at Diebold rigging the election as many left wing bloggers have already speculated? One, there's no evidence of that. Two, weren't these same conspiracy theorists claiming that Diebold worked for the GOP in the 2000 and 2004 elections? So now, Diebold takes orders from Hillary Clinton? Conservatives should file that information away and make sure to bring it up if and when the same people claim that the 2008 elections are rigged.
Could it be that Hillary just did a better job of getting out the vote? She may have, but a fantastic GOTV effort is still probably only worth a point or three at most. So, if a race is genuinely tied and one side has a much better GOTV operation, it may enable them to pull it out, but it won't overcome a 7-8 point advantage.
Well, what about a large block of undecided voters suddenly choosing to vote Hillary at the very last minute? Voters might break one way or the other as a race draws to a close, but they won't break hard or fast enough to cause that big of a shift. Moreover, it's difficult to believe that we didn't see more movement in the final polls if voters really were strongly ralling towards Hillary.