For those of you who respect Nate Silver (and I think most of us do, even when the analysis he brings is less than promising for our side), you're going to love what he has to say today about Gallup:
...its results are deeply inconsistent with the results that other polling firms are showing in the presidential race, and the Gallup poll has a history of performing very poorly when that is the case.
More...
This is an excellent article for the amount of depth Silver goes into regarding how he weights various polls and arrives at his predictions. He does give a fair amount of weight to Gallup, relative to other trackers, because of their sample sizes.
However:
Of the daily tracking polls, the Gallup survey receives the largest weight in the trend-line calculation. It uses a larger sample size than most other polls, and it has a methodology that includes calls to cellphone voters.
On the other hand, the pollster ratings are also based in part on past accuracy, and Gallup’s performance is middling in that department.
Here is a chart showing the relative weighting of each group of polls when calculating the polling average:
As you can see, Gallup's daily tracking poll accounts for a very tiny sliver of Nate's overall weighting. You'll also be pleased to see the enormous amount of weight given to state polls, and we know that Obama's numbers have been holding steady in key states. He goes on to note that Gallup's track record is not stellar when they are in disagreement with other polls:
Usually, when a poll is an outlier relative to the consensus, its results turn out badly.
You do not need to look any further than Gallup’s track record over the past two election cycles to find a demonstration of this.
I encourage you to go and read the whole thing, including a fairly detailed recent history of Gallup's poor showing. For those of you sweating Gallup's recent numbers, this will do a lot to put your mind at ease.
Oh, and Nate's summation:
It’s much more likely that Gallup is wrong and everyone else is right than the other way around.
[
Thanks to ItsSimpleSimon in comments.]