The latest Survey USA poll out of North Carolina (showing a 20 point McCain advantage) has already generated some disbelief among Pollster.com readers. There may be good reason for some skepticism. After all, less than one month ago, Survey USA showed just a four point lead for McCain. Is this newest poll evidence of a huge Palin bump? An outlier? Evidence of the "shy Tory factor?" Or have Republicans suddenly become much more likely to vote?
I'm not sure we can answer this question definitively yet, but we can lend some context to this poll. At about the same time as Survey USA was releasing the North Carolina result, they were also releasing surveys from Virginia and Washington, neither showing as dramatic a swing. In Washington, Survey USA showed a 7% lead for Obama in early August and now shows him up 4%. In Virginia, McCain held a 1% advantage in early August compared to 2% now. The Virginia result is particularly important since it comes from a state that neighbors North Carolina and shares some demographic similarities. If there was a Palin bounce in North Carolina, presumably we would've seen more of a bump in Virginia as well.
An examination of the cross tabs goes a long way in helping us to understand the North Carolina result. Fortunately, Survey USA provides us with the composition of their sample among various measures. Of particular interest here is their party break down. In early August, 46% of the Survey USA sample was made up of Democrats and just 33% were Republicans. In the most recent poll, the margin was essentially even--40% were Democrats and 41% were Republicans. In other words, in a survey conducted less than a month later, Democrats made up 6% less of the sample and Republican representation increased by 8%.
Everything we know about partisanship suggests that such massive shifts over such a short period are highly unlikely. In other words, it is not very plausible that North Carolinians became 6% less Democratic and 8% more Republican in less than a month. So, what are the other potential explanations? One is the "shy Tory factor" outlined by Nate Silver. According to this theory, it could be that Republicans simply weren't answering surveys at a representative rate a month ago because they lacked enthusiasm, but now they are excited to do so because of the Palin selection. Thus, the increased representation of Republicans in the more recent Survey USA poll may be the result of more Republicans agreeing to be interviewed. But if the "shy Tory factor" is in play, it seems like we'd see a similar increase in Republicans in other surveys. A related theory is that the Palin selection made a lot of Republicans in North Carolina more likely to vote, and since Survey USA presents results from likely voters, they are picking up this change by having more Republicans in their "likely voter" group.
The chart below compares the party composition in the August and September polls conducted by Survey USA in North Carolina, Virginia, and Washington (the numbers don't sum to 100% presumably because one can choose "other" or skip the question).
As indicated in the chart, there were small shifts in the partisan composition of the samples taken in Washington and Virginia, but nothing like what is evident in North Carolina. If there is a "shy Tory" effect or a mobilization effect, it does not appear to be playing out similarly across different states. Most significantly, there appears to be only small (and probably not statistically significant) changes in the composition of the electorate in Virginia, North Carolina's neighbor to the north.
Of course, we really cannot know whether this North Carolina poll is an outlier. But based on the dramatic change in the partisan composition of the North Carolina sample, without a similar change in Virginia, the most likely explanation at this point is that Survey USA may have just drawn a bad sample, one that over-represents Republicans and under-represents Democrats. At the very least, it would be useful if Survey USA can provide more explanation about why they think the partisan composition has changed so significantly in this recent poll.
UPDATE:
Just did a little more research and the following two points seem relevant:
According to the excellent North Carolina Board of Elections site, as of September 6th, 45% of those registered to vote in North Carolina are registered as Democrats and 33% are registered as Republicans.
In a July Survey USA poll, the composition of likely voters was 45% Democratic and 37% Republican. (McCain held a 5% lead in that poll).