Hm, maybe it's just because I tend to hang out around Obama voters, but that doesn't really sound right to me (or explain why the willingness to be called racist seems to have started well after Obama). I heard a fair bit of anti-Obama rhetoric, but it was all policy-based. Now, I have a very limited perspective--I'm just me--but if there was a "whitelash", it didn't show up very strongly in voting: Obama got 43% of the white vote in 2008, 39% in 2012. That's a pretty small shift--only a little bigger than the decrease in Hispanic (71% to 69%) and black (95% to 93%) votes. Asians were the only group with increasing support (62% to 73%). And it didn't show up very strongly in turnout: voter turnout among whites went down from 66% to 64%. Maybe there was some subgroup that was exactly like you describe--but it doesn't seem like it was triggered very broadly by Obama being black unless it took over four years for people to notice he was?

That it was brought on by Trump seems more plausible, but even then, Trump himself seemed at pains to avoid any public admission of racism: even while talking about the wall it was always about criminals or somesuch; when talking about banning travel from Islamic countries it was about terrorists; and so on. Still, the Presidency is a very powerful place from which to set trends, and the tacit acceptance without (much) explicit acceptance may have been enough.

It's not a very satisfying answer, though. For the war on truth, Trump's efforts were incredibly out in the open, and seem to have worked to an alarming extent: alternative facts, grabbing and pivoting the "fake news" moniker to be a weapon rather than a vulnerability, etc.. And you can see the fruits of that labor: the outlandish percentage of Americans who (claim to) believe the Big Lie.

So it's kind of hard to believe that this is the whole story, even if it plays a part. Now, if you're adding to that the fact that Trump was called racist all the time, but like every other charge against him, he found a way to not make it stick, that might be another key factor. If someone calls Trump a racist and it doesn't matter, who cares if they call you "racist" too?