Josh Chafetz asks the $64,000 question in American public opinion polling:

[W]hat, exactly, is the point of continually doing nationwide polls when all that matters are the states? I mean, I know nationwide polls are a lot cheaper, but just making up the results would be cheaper still, and only marginally more relevant.

Well, I don’t know that nationwide polling is truly irrelevant; the state-level poll results would be close to a simple linear function of the national polling number, although the effects of campaign advertising—concentrated in the “battleground states”—will cause divergence from linearity.

But due to statistical theory, and the closeness of presidential elections, you’d have to survey a lot more people to get accurate state-level data… realistically, a sample less than 500 per state is useless, which means polling 25,500 people (including D.C.) per survey—and you’re still getting a sampling error of ±4.5% per state. So the best that you can do is pretty much what’s done in practice—you do national surveys augmented by state-level surveys in states that are a priori believed to be close.