There is no secret to this. Pollsters don't spend time combing through records looking for clues as to who might be a Democrat or a Republican. They call a random sample, and any scientist will tell you that the larger a sample is, the more it is going to represent the population as a whole, as the margin of error becomes less. So with this poll, it looks like more Americans are identifying with Democrats than Republicans. In 2010, more Americans were identifying with Republicans than Democrats, and that could very well happen in 2012, depending on how Obama does.
In short, those who are complaining that more Democrats were called are putting up a red herring, and building a straw man, both at the same time.
I have a story for you, and it is true. My wife is very Liberal, and strongly identifies as a Democrat. In 2010, she complained that Rasmussen was purposely polling Republicans to make the Democrats look bad. I explained to her what I have posted in this thread. She wasn't buying it, of course. She IS a hyperpartisan. We have a good marriage, but when it comes to politics, we argue hard. And, sometimes, she just refuses to listen to reason. Some people in this thread are also refusing to listen to reason. Some are just like my wife, and some are even worse than my wife. To them, it is better to take the blue pill, and forget reality. The only thing in life that is important to them are the political parties that they are slaves to. To those people I say "You can keep the political slavery". I don't want it. Critical thinking, and intellectual freedom, is better than being a slave to any political party or ideology.
To sum it all up, the whole argument brought up by those complaining that AP took a biased poll can be summed up in one word....
Bullocks and dishonest.... Oops, make that 3 words.