In the 2012 election, as it became clear Barack Obama was polling better than Mittens Romney, it was claimed the polls were "skewed" in favor of Democratic candidates. Turned out that the polls underestimated Obama's margin of victory. But with Donald Trump trailing Hillary Clinton in similar fashion four years on, the "unskewers" are back.
The basic premise of the unskewers is wrong. Most pollsters don't weight their results by party self-identification, which polls get by asking a question like "generally speaking, do you usually think of yourself as a…." Party identification is an attitude, not a demographic. There isn't some national number from the government that tells us how many Democrats and Republicans there are in the country. Some states collect party registration data, but many states do not. Moreover, party registration is not the same thing as party identification. In a state like Kentucky, for example, there are a lot more registered Democrats than registered Republicans, but more voters identified as Republican in the 2014 election exit polls.
It's looking rough right now for Trump voters. Fivethirtyeight's Polls-only forecast gives Clinton an 87% chance of winning, and the electoral map (with a 364-to-172 landslide college) is plainly as much as it is possible for a Democratic candidate to gain under any circumstances.
If Trump spent the next 100 days running naked smeared in his own excrement, Clinton might pick up South Carolina as well