Are polls understating the support for Republicans?
Would you answer an unidentified person's questions about your preference?
Nixon had his "silent majority" at a time much like this one. The country was tearing itself apart over Vietnam and Nixon was the target of almost as much liberal vitriol and hate as Trump has been. The reason pollsters may be wrong again this time is that these "shy Trumpers" have been conditioned these last two years to keep their opinions to themselves so they don't get screamed at by their friends and relatives. There is a price to pay when bullies attack you for saying something halfway nice about the president. It's not pleasant.