This is because his model is a whole world to itself, and within that world the number of things that can change the probable outcome are declining every day. It is exactly like the program that calculates the odds of a Texas Hold 'Em hand winning or losing when you watch the World Series of Poker. Every time a card is turned over, if it doesn't materially affect the odds in favor of the challenger, the odds of the high-hand holder go up. This is because there are fewer cards left that might change the game for the low-hand holder.
Thus, Mr. Silver is to a large degree correct. If the state polls are correctly modeled, and if no new data out of line with the existing data is introduced, time is grinding away the opportunities for anything different to occur.
Nevertheless he makes a huge error.
But many of the pollsters are likely to make similar assumptions about how to measure the voter universe accurately. This introduces the possibility that most of the pollsters could err on one or another side — whether in Mr. Obama’s direction, or Mr. Romney’s. In a statistical sense, we would call this bias: that the polls are not taking an accurate sample of the voter population. If there is such a bias, furthermore, it is likely to be correlated across different states, especially if they are demographically similar. If either of the candidates beats his polls in Wisconsin, he is also likely to do so in Minnesota....That "sixteen percent" chance that the assumptions are flawed is entirely bogus. The model can account for sampling error of the "plus or minus three percent" variety; there's no problem with that because you can give it a percentage using known methods. But there's no way to know what the odds are that a flawed assumption is making the data itself unreliable.
My argument, rather, is this: we’ve about reached the point where if Mr. Romney wins, it can only be because the polls have been biased against him. Almost all of the chance that Mr. Romney has in the FiveThirtyEight forecast, about 16 percent to win the Electoral College, reflects this possibility.
That's the one thing that the model can't actually measure. Any attempt to estimate it is a completely unscientific guess. Investment bankers and pollsters are each a class: they make slightly different guesses, based on their position and what they want to achieve, but they inform themselves based on talking to and watching each other. If they're wrong, it's as likely as not that they're almost all wrong.
There's simply no way of knowing what that means for the model, because the whole model is built around data shaped by their assumptions. It's like trying to guess what the odds are that Elvis is still alive in a nearby possible world: there's no way to put a number on it, because the world in which Elvis is alive is a world built out of entirely different facts.
If the pollsters are biased, the facts of the true world and the facts of Mr. Silver's world are simply not the same at all. His model won't just be wrong in some way that can be estimated and worked into the model. It'll be so wrong that it simply can't be applied to the actual world. Any resemblance between his world and ours will be accidental.