I didn't realize the anti-immigrant sentiment had gotten to this level in so much of the population

I think the causation is backwards here, a lot of low engagement voters just assume that the best policies are somewhere to the left of whatever the GOP wants and to right of whatever the Dems are pushing for, but they keep thinking that even as the Dems move to the right.

Dems were thinking if independents saw that they'd respect the Dems willingness to compromise or whatever, but Indies saw that and just decided "Oh, I guess immigration really is a problem like the Republicans were saying all along, even the Democratic party thinks we need a border wall now."

e; an attempt at better phrasing