Bottom line, any poll winds up with an assumption...either created or by happenstance of response....on the weight of an election. IF Dem turnout is TEN percentage points more than GOP, THEN perhaps Hillary is way ahead. But that has no basis in reality. As the OP stated, Gallup has party Id EVEN. Now pollsters could use other methods, but it is reasonable to state they Party ID question is even.
In 2012, If I recall, The Dems only had a 3 point advantage in Party ID polling at this time of the year. So from that score, the GOP is already polling 3 points better than 2012.
It is true this election is going to be bit odder than any since the Perot elections, because some of the particulars are all over the map...like Trump getting big Latino and Black support in some polls, and not in others. Like Dems voting Trump and GOP voting Hillary.
Then you have the Sanders voters, and Johnson polling above what I personally think his name recognition is.
So this election could be a real watershed true shift election...meaning....none of the usual models are going to work, and if they do, it would probably be dumb luck.
I see this as globalism vs nationalism, and Hillary’s corruption vs Trump’s perceived personality issues. That has the potential to provide a very unusual set of voting patterns in Nov.
And we haven’t even made it to the debates yet...and Trump has yet to even start a traditional ad/local office campaign.
Throw in the leaks, and any pollster who is telling you they have this nailed is lying.
Good post. I think you got almost all of it. One thing that one should add is that the response rate to any poll has declined over the years and is now (according to PEW) below 10%. (NB. Very rarely do the polling companies tell you how many interviews they have started, or how many phone numbers they have called).
As I have written elsewhere some research articles indicate that the errors in the polling have not increased despite the reduced response rate. That may well be true, but one has to be careful. Any election where there is more reason for voters for one candidate to not divulge their choice can cause very large discrepancies in the polling results.
Given the media’s relentless pursuit of Trump we may (cannot prove it, but we may) have such an election now. In that case the polls are not worth anything.