Andy Gelman has an excellent post that almost constitutes a direct salvo in defense of the USC polls being conducted for LA Times. (NB: I think USC polls are still skewed somehow, but there are also important bits of information being lost in the usual polls that it provides).
The truth about elections and parties is simple: party id’s don’t change too much and people tend to vote party. If a poll shows a big swing in one direction or another, accompanied by a big swing in the partisan composition of the respondents, then the poll is probably capturing a big swing in survey response bias that correlates with partisanship. We want to know what the choice will be, conditional on all sorts of factors known to be correlated with the vote choice–and partisanship is indeed highly correlated with the vote choice.
In context of 2016, this is especially relevant since Trump’s unpopularity is especially acute among the Republicans–at least, those voters who are reliably Republican in most elections: wealthier white suburbanites. He is barely tied with Clinton among this subsample, according to various polls, and trailing significantly behind among the women among them. These are, in a sense, far more significant predictors of Trump’s defeat than how unpopular he is with the minorities, who tend to be overwhelmingly Democrats anyways.