This is independent of politics or partisanship or Trump-love or anything else. It's about why we humans love to categorize and stereotype people--and put people into two essential categories: "us" and "the other."
Our brains are built to use shortcuts--often called "heuristics." We bypass higher reasoning centers to form instant judgments about what is dangerous and what isn't. This is adaptive and positive for survival--for instance, a shadow passes overhead; Og whirls and presents his spear, without stopping to analyze what exactly it is that might be casting that shadow. Stopping to think might cost him his life.
Similarly, Og sees an unfamiliar human. He must judge instantly whether this other human is friend or foe--or neutral. He will look for signs such as what kind of clothing or weapons the other guys is carrying, or his hair or skin color. This is using a friend-or-foe heuristic.
This is so adaptive and useful that it has carried over into modern life. We see another person. Is he one of "us" or one of the "other"? We drop down to the heuristic of skin color. If he's Black and we're White, he's obviously not one of "us" and we should view him with suspicion. We do this unconsciously, a thousand times a day. Friend or foe? What signs or markers does he carry that he's of our tribe? Such a marker could trump skin color; for example, a Pittsburgh Steelers hat (he's obviously one of "us").
It feels good when we do this, regardless of whether we've drawn a positive or negative conclusion about that person. We've figured out a little piece of our world (even though we might not actually have done so, we think we have). We feel just a little bit safer.
Therefore, when we express racist attitudes and perform racist acts, we're satisfying a primal urge. We both identify and ostracize the "other" and solidify our bonds with "us." This feels good to a LOT of people.
So it's futile to think that racism is some kind of aberration or misfiring of the human mentality. On the contrary, it's fundamental to who and what we are. Now, that's not to say that our impulses can't be tamed and controlled: civilized. Og knew--or learned--that while he might have had the urge to bash the guy next door over the head and take his stuff, that was ultimately bad strategy. Likewise, we can learn to regard the guy next door as one of "us" (Americans) rather than one of "them" (Blacks).
But let's not kid ourselves that it's going to be easy. We know that (sadly) racism is a feel-good thing for a LOT of people. They won't let go of it so readily.