Identification causes trouble in that it brings along with it registration in several states, most notably Oklahoma. You still manage to find a trend, of course. In fact, it may be sharpened by the fact that there is a "softer" Democratic base to work from. But I don't think they're "honest" numbers to begin with.
I know what you're trying to quantify, and I'm just not sure it's possible. Even federal elections aren't totally federalized, if you catch my drift.
What if we looked at the states that have had Bush's approval numbers decline the least and most relative to 2004?