http://fivethirtyeight.com/features/missing-white-voters-cou...
The summary is that by far the largest unregistered cohort is those who are young, poor, and uneducated. And this election, the poor and uneducated group is largely in Trump's camp, while in past elections they may have split roughly evenly between Republicans and Democrats depending on other issues.
The other interesting thing is that the Trump campaign has done fairly little with voter registration drives, perhaps because of this image of it largely helping Democrats. Would be an interesting example of how hewing to common wisdom rather than data can end up hurting a candidate, if 538's analysis is true.
The tricky part about using data is that it takes time to build an effective data operation. It takes time to collect the data, it takes time to analyze it, and then you need a large ground organizational effort to actually turn that data into votes. The last day for voter registration in most swing states is Oct 11, barely a week away. Even if he's recently had a change of heart, it's likely too late to do anything effective.
[1] http://elections.ap.org/content/ap-interview-trump-says-big-...
[2] http://www.wsj.com/articles/trumps-big-data-gamble-146939531...
[3] https://www.wired.com/2016/08/trump-cambridge-analytica/
What? This is only true if you believe that get-out-the-vote (GOTV) operations are exclusive to Democrats.
Non-targeted get out the vote campaigns are always in favor of Democrats.