Perhaps most important is the group that voted in much larger numbers than in 2012: white voters without a college degree. (Trump won this bloc 63 percent to 32 percent.) Generally speaking, college graduates are more likely to vote than non-college graduates, even when controlling for race. According to the Current Population Survey, whites without a college degree made up 44 percent of voters who cast a ballot in 2012, and 58 percent of registered voters who didn't vote.
These may have been some of the "missing" white voters that RealClearPolitics Sean Trende has written about, but in 2016, they weren't missing. In the SurveyMonkey data, white non-college graduates made up 48 percent of 2016 registered voters who didn't vote, substantially lower than 2012. They made up 47 percent of voters. It's pretty remarkable that a group of voters that is shrinking as a percentage of the population made up a larger share of the electorate in 2016 than in 2012. But Trump made a clear appeal to this group, and some voters who stayed at home in previous years may have felt they had a greater voice in 2016.