A national poll of likely voters of sufficient size taken over a span of a week should be reasonably representative. Gallup has a pretty good track record on that.
Gallup’s final presidential polls since 1960 have generally hit the GOP number or slightly underestimated it. The last two elections Gallup was low by about 2 pts for the GOP candidate. The only one they were really high for the GOP candidate was in 1988 when they predicted 56-44 for Bush, and it ended up being 53-46.1. Conversely, in 1980, they estimated Reagan at 47-44 (8 for Anderson), and that election ended up being 50.8-41-6.6.
If there is an error, the error would be in the average as well.
For whatever reason, the polls seem to get much more accurate right before the election.
With the early voting and vote by mail, I suspect when you get close enough to the election, you are almost already getting an exit poll.