As Election Day approaches and President Donald Trump continues to trail Joe Biden by high single digits both nationally and in key states, their respective bases are buzzing with either hope or dread that "the polls could be wrong again."

In truth, public opinion polls are imperfect instruments, and there's always bound to be some degree of error, especially given the widely varying quality of the nation's pollsters. But Trump would probably need a larger polling miss than in 2016 to win re-election, and there's no guarantee a systemic polling error this year would run in his favor.

First, it's important to remember that in 2016, the final pre-election average showed Hillary Clinton leading Trump 46.8 percent to 43.6 percent nationally, according to leading polling aggregator RealClearPolitics. That wasn't too far off the mark: She went on to win the popular vote 48.2 percent to 46.1 percent, not exactly strong evidence that hordes of "shy Trump voters" refused to tell pollsters their true intentions.

However, an examination of state-level polling reveals a clear and crucial pattern: In both 2016 and the 2018 midterm elections, state polls chronically underestimated Republicans' strength in the Midwest and Florida, and underestimated Democrats' strength in the Southwest.