At the risk of restarting the MRP wars: For the last 3 models I've designed (midterms, primaries, now revisiting stuff for the general) trying to impute how a state will vote based on its demographics & polls of voters in other states is only a mediocrely accurate method.

It's a decent stand-in when you have few polls and weak priors. Our models do use it a little bit of it. But generally speaking, looking directly at the polls in a state is quite a bit more accurate. And often, so are simpler "fundamentals" methods (e.g. national polls + PVI).

Part of the issue is that seemingly demographically similar voters in different states may not actually vote all that similarly. e.g. a 46-year-old Hispanic man in California probably has different views on average than a 46-year-old Hispanic man in Idaho or Florida...

That's partly because there are likely some unobserved characteristics (maybe the voter in Florida is Cuban-American and the one in California is Mexican-American) but also because states have different political cultures and are subject to different levels of campaign activity.