The list of voting groups generally alienated by Donald J. Trump is long: Hispanics, women, the young, the college educated and more. How is it that he’s in such a close race with Hillary Clinton?

The answer lies with a group that still represented nearly half of all voters in 2012: white voters without a college degree, and particularly white men without a degree.

Mrs. Clinton is showing enormous weakness with this group. And these voters are supporting Mr. Trump in larger numbers than they supported Mitt Romney four years ago. It’s enough to keep the election close. It could even be enough for him to win.