General Discussion

The problem is exit poll weighting in general. That college-educated white category is flawed primarily because other demographics are wildly misrepresented. The net, according to Pew research, is actually an exit poll underreporting of Republicans, specifically white non-college educated voters. The headline is a very poor summary of the actual issue:

https://www.nytimes.com/2018/02/27/upshot/trump-losing-college-educated-whites-he-never-won-them-in-the-first-place.html

"This winds up biasing the rest of the survey because the exit polls are weighted to match the actual result of a far less educated country. In general, the exit polls underestimate Republican support, probably in no small part because they overrepresent young, nonwhite and well-educated voters. But this process leaves the underlying educational bias of the sample intact, and the result is that Republican-leaning voters are given more weight to compensate for an electorate that represents Democratic-leaning voting groups.

In the end, the exit poll overestimates Republican support among most demographic groups, including well-educated white voters, and it overestimates the number of voters from Democratic-leaning voting blocs, like young, nonwhite and well-educated voters.

Those two facts go a long way toward explaining why there were so many Democratic-friendly analyses of voting demographics and polls over the last decade. The exit polling made it seem as if the Republicans, in a diversifying country, could not win with big gains among older, white working-class voters."