So what kind of windowing average do they use for the distribution? A loaded coin would have a consistent bias that the average would converge on. But for a signal that can't be modeled with an equation, you'd have to window the last N elections or something? And what about Reps that vote Obama or Dems that vote Romney? Is there a chain of Bayesian functions for the different demographics?
I think you are overthinking things. The assumption made is that on any given day x% of the population will vote democrat, y% will vote Republican, a z% won't be decided yet.
Now, if I could call every single person in a state and ask them how they are going to vote, I'd know x, y and z with certainty. I can't do this, so instead I can call a few thousand people at random and ask them. Now I ask, what values for x are consistent with the answers I got when I called? So instead of a probability of getting heads when I flip a coin, I have a probability of getting "I'm voting for whoever" when I call a random person the phone.
This is the simplest polling, and requires nothing of the last few elections as an input. Its just attempting to suss out what's going right now.
Now, the simplest version of correcting for demographics goes like this- let's say I call 1000 people, and I find out that 82% of people are voting for X and (18% are voting for Y, no undecides). However, I realize that I've gotten unlucky in my sampling and 90% of the people I've called are women. Further, the way women and men are voting is very different- 90% of women are voting for X, but only 10% of the men!
Here, I might be tempted to correct for demographics- I'd say 'ok, I have estimates of how men and women vote, but my overall sample is not representative. So, I'm going to go ahead and say in the actual population only 50% are voting for X(0.50*0.10 (men) + 0.50*0.90(women)). Now- this correction DOES effect our sample error- I leave it to the interested reader to figure out how to set the sampling error- but the why is simple. We have a much smaller sample of men, so a larger error in this case.
A more difficult question, I imagine, is what to do if your sample comes back 40% democrat, 30% republican,30% independent, or something of that nature. The problem is that democrats are very likely to vote Obama, Republicans very likely to vote Romney, so correcting a demographic imbalance can end up making your poll worthless- if you always correct to nearly 50/50 dem/rep the vote is alway going to be nearly 50/50. Also, people probably switch between a party identification and independent at a whim. If you are a Republican live in San Francisco or NYC and expect democrats to dominate, you might be more likely to indentify as independent so you don't feel like you were on the losing side. Same thing for democrats in Houston,etc.
Now when we aggregate polls we have to be careful because people's opinions change over time, so we might only want to aggregate the most recent polls, or use some model to try and predict how people's opinions change,etc. We can, in principle, make it as complicated as we like, but the underlying idea is very simple.