Just because some Republicans are racist doesn’t mean that all (or even most) of them are. In fact, no political party is immune to racism because it is based on natural human traits that have been bred into us over millions of years. Xenophobia is one of those traits, and it sometimes serves social groups of organisms well because it promotes harmony within the social group and tends to keep us from interacting with potentially hostile groups. But with the evolution of human intelligence, it has become increasingly difficult to set people apart based on arbitrary things like skin color.
We are still in a transitional phase, so sometimes our ancient heritage (fear, rage, xenophobia) conflicts with our newfound intellect (science, art, medicine). We haven’t evolved beyond racism yet, but we have reached the point where we recognize these natural feelings as archaic and barbaric. Consequently, most people aren’t aware of their racial bias and the rest are smart enough to know they should hide it. And while some of the more outspoken and visible closet racists may align themselves with Republicans, they do not represent the party or the vast majority of people who call themselves Republican.