The poll numbers between Clinton and Trump are narrowing. I think there are two simple answers for that.
A small group of Conservative politicians in Great Britain decided they wanted out of the European Union, and proposed a vote. Many of the anti-Brexit voters were symbolic votes, cast by people who never thought it would pass. It did. Now those voters are regretting it. Brexit was the breakfast of chumps.
That's happening in America, with Donald Trump taking the place of Brexit. The people casting the "protest" vote have, of course, been manipulated by the ultra-rich. The "protest" vote is a vote for a continued redistribution of wealth from the middle-class to the ultra-rich. Period. All the rest is click bait.
The second reason for the narrowing of the polls is the anti-Clinton vote, which is going to Trump. That voting block was also started, like Brexit, by a relatively small group of partisan Conservatives. But it got its legs, during the campaign of Bill Clinton, from the New York Times, Fox News and hate radio. The drag-along "Main Stream Media" picked it up and ran with it. They're still running.
The success of the anti-Clinton campaign has been built on perseverance. The Times, Fox, hate radio and Conservative operatives would invent a lie, and then repeat it over and over until it became a Big Lie. Citizens don't generally have the time or inclination to investigate, and so the lies become "common knowledge."
Trump does have some actual pro-Trump supporters, of course. My guess is that at least 20% of people who will vote for Trump are actually voting for what they imagine his beliefs to be.
The rest are "Brexit voters" and Republican dupes.