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Today’s Polls, 8/31

Both daily tracking polls now show Barack Obama moving slightly off his convention peaks. Gallup now has his lead at 6 points — down from 8 points yesterday — whereas Rasmussen has it at 3 points, down from 4 points yesterday.

Under typical conditions, we would expect the convention bounce to slightly increase, rather than decrease, as we headed into the weekend after the convention. As such, our model now regards Obama as having a slightly-below-average convention bounce. Obama now leads in the Gallup and Rasmussen trackers by an average of 4.5 points. In the week before the convention began, he had led in those polls by an average of 1.2 points. So, that represents a convention bounce of 3.3 points — below the over-under number of 6.

On the other hand, there has never before been a circumstance in which the opposing party’s VP has been named immediately after the convention, an event which often produces a bounce of its own. If a typical VP bounce is 5 points, and McCain has received that VP bounce by having named Sarah Palin to his ticket, Obama can be regarded as having had an above-average convention bounce (an 8.3-point convention bounce, counteracted by a 5-point McCain VP bounce).

All of this is academic, really, since all of these bounces will fade. I just wanted to emphasize the point that, because our model adjusts for the convention bounce but not the VP bounce, it is probably lowballing Obama’s numers a bit at this point. By this time next week, when the GOP has had a convention of its own, we will be in a better position to evaluate the state of the race. I would recommend that you take our numbers with a grain of salt in the interim.

Nate Silver is the founder and editor in chief of FiveThirtyEight.