Race theory became prominent in the 19th Century after the liberal revolutions across the Western world. Clearly races exist in that there is a continuum in which a "white" is vastly far away from a "nigger".
However, does this mean that forming strict categories out of this continuum is a good way of organizing polities or even particularly feasible today? Nationalists (also a product of the enlightenment, if you understand what the word refers to, feudal kingdoms were in no sense "nationalist") can't even agree on whose white? Are slavs white? Are the Irish white? Where do you draw the line? Some want to draw it based on Europe, whereas others hate the idea of European unity and consider certain Europeans to be disgusting (see Brit/pol/).
There's also the issue of drawing the line in terms of percentages. Is someone "99% European" white? Is someone "80% European" white? 50%? And wouldn't it depend on who they mixed with? Do we just go with the system the German National Socialists used or come up with some other system?
What exactly is the point of all this autism when there's a really easy way to tell who is inferior and deserve to die, and who is superior and deserves to live?
A sub-saharan African is always going to fail at things a European will succeed at. The entire point of using the racial categories in nationalist politics is to exclude those who degrade civilization. The problem is that you get into trouble when you have to decide what the exact boundaries are centrally by decree.
Here's a novel proposal. If you want the results of a good and noble society, you are better off raising the bar up from the degraded level of this society.
Enforce tougher laws, and bring in harsh criminal penalties on a fair basis. The good will avoid falling foul of this behavior and the inferior will fail and be destroyed. Usually the good will be a certain percentage "European" and usually the bad will be some high percentage something else, but the end goal should always be structuring the law to remove degeneracy in an organic manner, rather than decreeing from the start, centrally, and bureaucratically what identities are and are not degenerate.
Behavior, not race should be the prime standard, because behavior will encompass race anyway. If we want rid of stupid impulsively violent high time preference people, then harsh criminal laws, and no welfare state or protection for the weak will do just that, and the wide distribution of solutions to the problem will produce much better results than a central racial decree about what % of blood disqualifies you from society.
Racial facts are instructive to policy, but if they are made the primary motivator of society, you simply create an autistic collectivism that will destroy itself from within. The racial concept is less egalitarian than the idea of the human race, but it also contains a kind of stealth egalitarianism qualified within the boundary, since it starts from a deontological basis of "white is right" rather than a consequentialist basis in which harsh selection is imposed and we see who is best. The facts of history show us that Europeans were the most the successful, but it is important to avoid arrogance and putting the cart before the horse. Pursue civilization first, and the best race will meet the task, but pursue race first, and you risk a decline in standards.
Racialism is only to the right of today's self-identified left, but it is not a true reaction and solution, since you are arguing within the same framework they are. You assume merit rather than pursuing it.