As noted, in this thread and elsewhere:
Good polls will report factors like party identification in order to adjust the poll to better match the public at large. E.g. if the data shows that 49% of Americans are registered Democrats and Dem-leaning independents, and that 44% of Americans are registered Republicans and R-leaning independents, they will adjust the survey results to better match those figures.
Polls that are consistently inaccurate usually get called out. E.g. Rasmussen tends to provide rosy numbers for Republicans in some elections, so the aggregators try to keep that in mind.
More sophisticated aggregators (like 538) will sometimes try to readjust for poll weighting, and throw in their own special sauce (such as noticing correlations between states, often based on geographical quirks.)
And yes, those numbers are largely accurate. Gallup, for example, has tracked party identification for years, and Dems outnumber Republicans. Fox may rely on Gallup or external polling, or run its own party identification polls.
There's far too much bitching and personal attacks going on in this thread. Please stop and stick to the topic. And that means you, Mycroft and you, BD. And everyone else who's doing it. 