The community consensus is: we do not tolerate promoting racism here. And as elsewhere Ignorantia juris non excusat.
Apparently necessary prolegomena:
What is "race"?
"Race" is an either biologic or biologistic system of categorisation or taxonomy for grouping life forms. This was also applied to human pouplations but came to be recognised as scientifically obsolete since the 1950s, as biologistic determinism proved incompatible to explaining greater intragroup variation to lesser intergroup variations in human populations on the one side and shifting weights within the nature-nurture debate that lessened emphasis on genetic determinism for individual human development.
What is racism?
Racism is an attitude, belief or ideology according to which people are categorised and judged as so-called "races" on the basis of a few external characteristics – which suggest a common descent. The characteristics used for differentiation such as skin colour, body size or language – but also cultural characteristics such as clothing or customs - are interpreted in their biologistic meaning as a fundamental and determining factor of human abilities and qualities and classified according to their "value". Racists generally regard all people who are as similar as possible to their own characteristics as superior, while all others (often graded) are discriminated against as inferior. With such race theories, which are allegedly scientifically supported, various actions were and are justified, which contradict the general human rights as understood and applied today.
Basically, racism is taking a number of minute statistical differences in either behaviour or biology of groups of people and declaring that without basis in sound evidence as essential and deterministic to individuals and populations alike, always carrying a value judgement that is designed to degrade others. As the biology of race is so weak it cannot be a meaningful foundation for justifying the racist outgrouping and dehumanisation that appears sociologically or politically. The human rights issue is just the icing on the cake of scientific dishonesty, although some might prefer to start their reasoning from there. The result will be the same.
There are just tiny problems here when applying this to History.SE: what exactly is "promoting racism" and how do we deal with ignorant (see above) postings? I think there can be cases when a posting can be improved to comply to guidelines; from "can be read as being racist content" to "tackles the topic of racism but is not racist itself". Almost 'naturally', there will be cases of racists posting racist dreck that cannot be improved and posts that should just be deleted, on sight on this site.
There was a new (I think) 'borderline' case on main: https://history.stackexchange.com/questions/49045/what-is-the-most-advanced-scientific-basis-for-anti-russian-sentiment (In hope that it gets edited, the following will refer to revision 1 in the edit history of that currently deleted question)
Statistics: estimated reading time 4 minutes, 6 downvotes, no upvotes, 8 links showing research but with questionable context given, put on-hold (within less than 3 hours), mod deleted (for racist content).
Mod Mark did comment:
There is no scientific, rational, basis for racism. The statement that X "are incorrigibly inferior and unable to improve." is racist and cannot be supported by any scientific argument.
He wrote a sentence that is completely valid, now, and reflects the community consensus. However, this is probably not what this question was really about. Great minds like Aristotle, Kant had some ideas now considered racist and thought of that as scientifically valid (speculation/reasoning). Both philosophers would have a hard time lecturing that at face value at any university now. But as far as I understood it, the question on main is about a setting from 1920–1950.
In that timeframe some held the view that "X 'are incorrigibly inferior and unable to improve.'" Since most of now think of this as a racist statement and that it cannot be supported by any scientific argument, OP probably wants a character that sounds as if he would have scientific arguments that seemed to support his position.
The way I read the question, it was about historic racism. About how to incorporate a character (who is essentially a racist) as a believable character. That is different from portraying a racist theory within a novel and that that theory would be believable now, or that it could be used for that. (If that kind of promotion is indeed the intention of the example question then I am seriously in error about that example and the rest of this answer immediately does no longer apply to the example question any more as a whole. Please regard the following then and therefore as a general comment on the situation.)
That would mean that the character at least hints at by name to a once widespread idea/concept/theory from the racist realm. This might be a primitive parole, heard in a speech, read in a book or newspaper, then regurgitated by a character in a novel, who would then look racist and unintelligent to wel educated readers. But racism and dumbness are not perfectly proportionally correlated and they never were. Racism is more of a belief system than something to understand –– but also, like other belief systems, something intelligent people tend to somehow rationalise, however misguided.
We now know, or at least should know, that racism is inhumane and inherently stupid, from a scientific viewpoint. For a character to be believable as racist and not coming across as totally stupid a modern author might want to resort to that character adopting scientific concepts that were once used as reinforcing arguments for the basically unrelated but foundational outgrouping mechanism that is racism. These scientific concepts might now be completely outdated, disproven or laughed at or whatnot. They are not taught in that way in good schools any more. And that makes it difficult for a modern fiction writer to go beyond that what everyone should know now.
Mainly, I think that it this kind of context that was missing from the original question. Assuming best intentions from the OP I guess then further that a non-native speaker did write some parts down that at least look racist to an American, or if just taken at face value: parts that even are racist (guessing: without OP even realising it). Most readers thought: "Aha, a racist needs better racism for rambling in the streets and on SE!" But I hope it is still discoverable in the text of the deleted question that they should have read: "Oh, what a valid but difficult idea and how horrible this is presented. It invites misunderstanding, let's improve it!". The key is that the motivation should come out clearer and that the intent is really the absolute opposite of racism: not explaining (past) 'racism as good' (still) with 'science concepts', but explaining that even scientifically minded persons succumbed to the flaws of thinking, group dynamics and feeling at that time.
In this case, how should OP signal his intentions better? More careful wording, proper grammar and definitions for the intended meaning, resulting in a clear distance from what characters say and what might be read as what the posters holds as true or even some kind of "a truth universally acknowledged".
While being very glad that this din't go 'hot' (having all the hallmarks of a Q for that) I think the downvotes are quite extreme. The Q is not a good one in presentation now, that's a given. But aren't most readers just mislead for the intentions OP had with it (despite being discoverable in the text)? –– Anyway, as I think that this can be made good, I'd plea for a delay should deletion votes come in quickly.
To re-iterate: The way I read it: Promoting racism is not the intention. Intention is (well, I hope should be?) Fictional character (most probably not the hero (good guy) should be historically accurately, or believably, be painted as racist, and argue for his position without sounding too idiotic, unbelievable. A problem well educated writers might face easily!
I guess this is much more about historical racism, than intended as being promoting racism itself, now. But the wording misleads most readers. In parts it actually is rebuilding racist ideas without connecting that back to the thoughts laid out above.
To be clear: I may very well be in complete error about the intent and actual content as intended in the example question. This is no unconditional plea to get it undeleted. If I am wrong about the example it should stay deleted.
That kind of questions could be improved, they can be improved, they should be improved. Questions about racism are difficult, to say the least. But they should not be a total taboo. They are important. We do not want to promote racist content. We do not want to foster racist ideas. But one method to achieve that might be to have content that describes how racism appeared to have worked and appears to work in the minds of those who held those views.
As another answer here wrote:
Race is an unacceptable theoretical category as it has been vigorously excluded from the scholarly tradition.
The unappetising thing about our history is that race was an acceptable theoretical category for perceived or imagined differences between human groups and it has been vigorously excluded from the scholarly tradition only fairly recently, in some parts of the world.
In my personal opinion, just naming a group of people ––(example: skin colour varies, I see that, but what follows from that? Not much. Properly educated "blacks" can get a noble prize and properly miseducated "whites" can be so incredibly stubbornly stupid as to necessitate this post. Change the colours around to your liking if you have to.)–– in any way should be unproblematic, regardless of what that name is. But giving out certain names is considered bad etiquette (I agree, gruntling about the humanity) and even a kind of racism itself (no full agreement). The common ground for what I would even opt to enforce the etiquette reason here is that those names deemed unfit for conversation is that these tainted names are symbolic communications that are meant to or even do carry the deterministic, disrespectful and degrading value judgements that are racist.