I'm sure there's a very small percentage of people who don't decide who they're going to vote for until they get in the ballot box. In the case of Diaper Dotard, that percentage has to be very small because he's been around so long and people who aren't going to vote for him will know so far ahead of time. The more pressing and unpredictable issue for a campaign is getting their voters to take the time to vote.
Keep in mind that for every poll you see, there's not only a sampling error (the +/- 3 (or so) percentage points based on whether the sample being polled is a match to the entire voting population) but other built-in errors, such as whether the sample polled are actually likely voters.
There are barely any high-quality state polls anymore. Good polling is expensive, and our media has been captured by corporate interests which are for the most part only concerned with profits. That's why there's so many shitty national polls. It didn't used to be this way. For example, Gallup used to do public polling on behalf of media clients but there's no longer demand for the type of extensive pre-election research they'd do in the public sphere.