We have seen a shift toward Mr. Obama in the polls since the Democratic convention. It appears that if an election were held today, he’d win it by somewhere in the neighborhood of four or perhaps five percentage points.Remember, the first assumption should be uniform swing. If Ohio and Florida figure to be dead even if the national race is dead even, then they figure to show a five point lead for Obama if he's up five nationally. Indeed: if that's the situation, then a new state poll showing uniform swing isn't new information about that state.
If Mr. Obama is ahead by four to five points nationally, we’d certainly also expect him to post his share of leads by about that margin in swing states. Because of statistical variance and differences in methodology, some of the numbers are going to be a little bit better for him than others. But the consensus of the data ought to quite strong for him.
Now, we know that we don't really have uniform swing, and in a very close race it can matter a lot if Ohio, say, swings a bit more and Utah swings a bit less.
If it's not a close race, however, that stuff just doesn't matter. More than that: we're still more likely to be mislead by a state poll that's a bit off, especially a single state poll, than we are by just looking at the national polling averages and just assuming uniform swing. That's going to change some over the next couple of weeks, as we get a lot more state polls (meaning that we'll start having meaningful averages of state polling independent of the national polls), but we're not there yet.
Perhaps, at the risk of redundancy, I should explain a bit more. Say we get an Ohio poll today showing Obama +5. Well, what we want is to make an Ohio polling average. But if we're only getting one Ohio poll a week, then the previous one might be from right after the Democratic convention (maximum Obama bounce!) and the one after that from before either convention. So you can take a simple average of them, but that's pretty useless because what we want to know is opinion in Ohio now, and the other two would be dated. So our choice is just one poll (lots of uncertainty) or averaging using outdated polls (meaningless for assessing now).
The best of the polling aggregations sites (Silver very much included) can do a bit more -- they can take each state poll and adjust for how it compared to the national average at that point, thus deriving an estimate for a state that should be a bit more precise than uniform swing. And yet, I'm pretty skeptical that we can extrapolate forward from June to November when it comes to that. Given that once we get beyond a three point national lead the state swings become totally irrelevant, and even with a one point national lead the state swings are probably irrelevant, I'm really just fine with holding off until just after this point in the calendar before I pay any attention to the state estimates at all.
And: nice catch!