Caution re New National Polls

John McCain clearly has benefitted so far from his party's convention and his choice of Sarah Palin. This is reflected in mostly dead-even national polls, sharp increases in the enthusiasm of Republican voters and gains among white women voters.

At the same time, changes in the number of Republicans and Democrats surveyed in the most recent national polls may have affected the results. From the Huffington Post:

"Emory University political scientist Alan Abramowitz is highly skeptical of the new Gallup, USA Today and CBS polls. About the latter, which showed a statistically insignificant two point lead for McCain, Abramowitz said: "One reason for the dramatic difference between the two recent CBS polls is that the two samples differed fairly dramatically in terms of partisan composition. The first sample was 35.2% Democratic, 26.2 percent Republicans, and 38.6 percent independent. The second sample was 34.9% Democratic, 31.1% Republican, and 34.0% independent. That's a change from a 9 point Democratic advantage to a 3.8 point Democratic advantage. That alone would probably explain about half of the difference in candidate preferences between the two [CBS] polls."

This must-read article for poll watchers points to the sharp increase in Democratic registrations, of more than 2 million in the 28 states that track party affiliation, while Republican registrations in those states has gone down about 350,000 during the same period.

- Richard Holcomb