Now, the polling profession’s defenders will insist that they don’t weigh poll results by party, they never have weighed poll results by party, and they’re never going to start weighing poll results by party, because we can never know what the partisan makeup of the electorate is going to be before Election Day.
We may not know the exact composition of the electorate on Election Day, but do have a historical perspective and a ballpark sense of what is realistic. If you consider 2008 to be the perfect storm for Democrats, and the “ceiling” for the percentage of voters who self-identify as Democrats, then most of the poll samples in swing states in recent weeks will strike you as weighed far too heavily in favor of Democrats.
Then again, perhaps more voters identify themselves as Democrats today than they did in 2008, when the country was gripped by Bush fatigue, Wall Street was melting down and taking the economy with it, the country was swept up in the euphoria of hope and change, the historical nature of voting to elect the first African-American president, and so on. I haven’t seen a really compelling argument as to why that would be the case, but it is theoretically possible.