When I look at a poll’s sample, and conclude that the sample includes a lot more Democrats than Republicans, I’m not quite saying that it’s an ironclad, take-it-to-the-bank guarantee that the poll is overstating the performance of the Democratic candidate on Election Day. From cycle to cycle, the partisan makeup of the electorate changes. It shifted three percent in favor of the Democrats from 2004 to 2006. It could, and in fact probably will, move further in the Democrats’ direction this year; the question is, how much?
I looked at the pollsters’ assessments of the electorate in their final polls in 2006. The ones closest to assessing the Democrats’ three percent advantage last time were Fox News/Opinion Dynamics, Pew Research, and Time magazine.
The last national polls by these organizations this year put Obama ahead by 9 percent, 14 percent, and 6 percent, respectively. But interestingly, they all used a fairly modest partisan breakdown in the electorate.
Fox News/Opinion Dynamics split 43 percent Democrat, 37 percent Republican, 16 percent independent among likely voters; it was 43-36-16 among registered voters.
Pew split 38.3 percent Democrat, 32.3 percent Republican, 29 percent independent.
Time split 37 percent Democrat, 29 percent Republican, 27 percent independent. (They also tried to push the independents by asking which way they leaned, and the sample split 47 percent Democrat, 38 percent Republican, 15 percent independent.)
So the Democratic advantage in these three polls – again, the ones that got the electorate’s partisan breakdown closest last time — is 6 percent, 6 percent, and 8 percent (or 9 percent with leaners).
So that’s something to keep in mind when examining the polls as we get closer to Election Day, and it’s worth keeping in mind for the state polls. D.J. Drummond looked at the partisan breakdown of the voters who showed up in each given state in 2006 and contrasted that to SurveyUSA’s batch of state polls:
Missouri: R+1 in 2006, SUSA using D+7, 8 point variance
North Carolina: R+1 in 2006, SUSA using D+5, 6 point variance
Pennsylvania: D+5 in 2006, SUSA using D+19, 15 point variance
Indiana: R+14 in 2006, SUSA using R+1, 13 point variance
Nevada: R+7 in 2006, SUSA using D+6, 13 point variance
Colorado: R+3 in 2006, SUSA using D+9, 12 point variance
Iowa: R+2 in 2006, SUSA using D+10, 12 point variance
Virginia: R+3 in 2006, SUSA using D+9, 12 point variance
Ohio: D+3 in 2006, SUSA using D+13, 10 point variance
Now, it’s not unreasonable to presume that the electorate in 2008 will be more Democratic than it was in 2006. But by margins like this?
The Axis of Right looks at polls in Virginia and Pennsylvania and finds similar results.