They already know, they don't care, because that specific belief is not rooted in reason or rationality.
> Going back to the original quote, you need to see that it's not calling all voters a particular thing. There's a simple Venn diagram, one circle of racists inside a larger circle of a particular block of voters.
> Educating people out of racism, and removing racism from your society, will change votes as racism is only one aspect of a person's beliefs.
I somewhat agree with the first part[1], but vehemently disagree with the second: I don't think that eradicating racist thoughts will move the needle on who gets elected, as there are, IMO, simply too few racists around to influence an election.[2]
[1] IOW, I don't believe that education will change a racist's belief, but I do see value to society in eradicating discriminatory stereotypes and discriminatory actions, of which racism is merely one.
[2] There aren't even enough racists to form a party of their own, so I doubt that them moving from red to blue is going to be any difference from statistical noise.