At least Gallup was able to identify a shift in party identification toward the Republicans after the GOP convention. Zogby insisted on weighting his polls to a 3-point or so demo advantage, since that was how voters in 2000 identified themselves. The problem was that on voting day, there were actually 5% (if my memory serves me) more Republicans voting than democrats. Gallup does not weight for party ID, which is what enabled them to notice this sea change.
However, those numbers are misleading.
There's a sizable percentage of registered or self-described "Democrats", who will never pull the (D) lever for any national office, especially that of president of the United States.