There is talk among Republicans, and some trepidation among Democrats, that Donald Trump could benefit from a silent vote. Although these voters aren't captured by polls, the privacy of a voting booth or a mail-in ballot will allow them to vent their anger and resentments.
The theory holds that in some circles it's not respectable to publicly support the inflammatory New York billionaire, but it's easier in private.
This is a variation of the so-called Bradley effect: In several instances over recent decades, white candidates have outperformed polls when running against a black opponent. Something similar was at work in the U.K. referendum on exiting the European Union. To the surprise of financial markets and bettors, the "leave" camp won the June vote, which was interpreted as an expression of discontent with the elites.
Similarly, in the U.S. presidential election, former House Speaker Newt Gingrich, a Trump supporter, claimed that "the average citizen will not tell pollsters the truth."