Industry standards put polling demographics as close to party registration as possible. For example, historically since FDR and WWII, about 38-39% of the populace was democrat and about 33% was registered Republican. So usually demographics were modeled after this. However, there has been a slow shift over the decades until noticably during the 90's it was clear that GOP registration has risen to 35% and democrat registration fallen to about 37%. This has continued until 2004 where it is about even now at 36%.
But the liberal MSM media continues to poll using post WWII models because it favors democrats and makes them look stronger. Scott Rasmussen has continued to add in new factors such as present day party registration and that is why he nailed the election in 2004, and those who did not, found themselves mystified why Kerry had lost so bad.
That is some interesting information, thanks for sharing. I imagine the shift is not really a shift in demographics but a slide to the Left by the democrat party that accounts for the biggest portion of that change downward. Wasn't it Reagan who said he didn't leave the democrat party, they left him?