Gallup's Frank Newport apparently felt the need to correct all the misinformation floating around about polls and party ID. In a column published yesterday, he offers a straightforward debunking of the right-wing idiocy.
Simply put, party ID is not static, but goes up and down with a candidate's popularity.
We know that party identification moves over time -- sometimes in very short periods of time, just like other political variables. Generally, if there is a political tide toward either of the two major parties, all questions we ask that are of a political nature will move in that direction. This includes the ballot, job approval, party identification, among others.
So, it would not be surprising to find that if Barack Obama is enjoying a surge in popularity in any given state, that surge will show up on the ballot question, on his job approval measure, and on the measure of party identification. So, data showing that Obama is ahead on the ballot in a specific state poll and that Democrats have a higher-than-expected representation on the party identification question, are basically just reflecting two measures of the same underlying phenomenon.
In other words - and this is something every "skewed poll" conspiracist out there seems to misunderstand - figuring out the partisan split of the electorate is one of the findings of a poll, not something that should be predetermined. Saying the poll included too many Democrats is basically the same as saying Obama is beating Romney by too many points.
More broadly, it doesn't make much sense to question poll results based on a particular assumption (the Democratic composition of the electorate must be at x level), when the whole purpose of polling is to tell us whether our assumptions are correct in the first place.