I had an aunt who the pollsters just loved. They phoned her often for about a five-year period. She happily answered their silly questions--and she had opinions on everything.
Except she never voted in her life. Making the trip to the voting station and cast her X was just not worth it. It's scary to think that public policy was being created by people like my aunt.
Polls can never ascertain the willingness of people's motivation to make a trip to the voting booth on election day. In this sense, polls are failing miserably. Add in that land phone lines are disappearing, one has to wonder who the pollsters are really talking to these days.
Election campaigns are about identifying the soft support of candidates/parties to ensuring these voters get the voting station on election day. A good "ground team" can turn a close election (which USA has a propensity to have many close elections). I tell anti-Trumpers on Medium that the best way to remove Mr. Trump is to join the local D ground team and put some effort in that task, rather than publishing another anti-Trump article. But . . . .
And election campaigns are also about convincing the soft support of the other side not to vote. All this negative advertising does work!