In the interest of fair debate, I offer some suggested definitions.
Nationalism: the knowledge that without a unifying culture composed of values, language, customs and heritage, a nation becomes unified by political policy and manipulated by the memes of advertising, politicians, marketers and so on. Nationalists recognize that pluralism and multiculturalism not only have failed historically, but inherently Do Not Work.
Racism: the belief that without a unifying culture, nations should be composed of mixed races, and that some should be dubbed superior and some inferior. In the racist view, nations can be unified by political belief, stocked with culture by marketers, and social rank can be determined by race and income. As a result, racists see it as necessary for people of each race to detest and manipulate each other in order to re-affirm social hierarchy.
So it seems to me.
Most people don't like the idea of social hierarchy. They love the idea that they can be anyone or anything they want, but we all know this is fantasy. No matter how hard I try, I will never be good at anything I want to be; I have some abilities that are innate that allow me to be good at certain things, perhaps even excellent at one or two. Would Beethoven have made a good politician or economist? No, but Alan Greenspan would have been a poor composer, also (I'm assuming Greenspan is a good economist).
Social hierarchy is perceived as most threatening when it deals with the inherent abilities of different people or groups.
The best solution, in the Nationalist view, is to stop the endless fighting and to separate into groups of similar people.
In the Racist view, separation is not the goal, but making sure that others know their place -- and clean the toilets of those dubbed superior -- seems to be the goal.
It's an interesting and under-discussed split.