The concern is obvious in this chart, which shows House exit poll data broken out by race and education. (We use education as a proxy for class because it correlates with income levels and isn't subject to shifts from inflation.)
The higher up the line on this chart, the better Republican House candidates did with the voting population. That light red line that's cruising along above the dark red one represents the shift toward the Republicans by white non-college graduates. The gap between college-educated whites and non-college graduates is the widest it's been -- and to the GOP's benefit.