Is It Defamatory to Falsely Call Someone a Racist, White Supremacist, Socialist, or Communist?

No, because courts conclude that those terms are just opinions and name-calling.

A lot of derogatory, even unfairly derogatory, criticism is treated as pure opinion, and thus not legally actionable. For instance, claiming that someone's appearance in some video (especially one that you link to) reflects a "smirk" (rather than a pained smile) and stems from racism, is likely to be seen as an opinion: A reasonable reader would understand it as the poster's subjective judgment about the video subject's motivations, and thus as speculation rather than an assertion about provable fact. To quote a nice summary from a recent federal district court case,

Statements indicating that Plaintiff is racist are clearly expressions of opinion that cannot be proven as verifiably true or false. While there appears to be no North Carolina court expressly addressing this issue, many courts in other jurisdictions that have faced the issue of defamation claims based on accusations of bigotry or racism have held the statements to be nonactionable statements of opinion. See, e.g., Stevens v. Tillman (7th Cir. 1988) (holding that neither general statements charging a person with being racist, unfair, unjust, nor references to general discriminatory treatment, without more, constitute provably false assertions of fact); Standing Comm. on Discipline v. Yagman (9th Cir. 1995) (holding that calling a judge "anti-Semitic" was a non-actionable opinion); Ward v. Zelikovsky (N.J. 1994) (accusation that plaintiffs "hated Jews" nonactionable); Covino v. Hagemann (N.Y. Sup. Ct. 1995) (dismissing defamation claim based on statement that plaintiff was "racially insensitive," observing "an expression of opinion is not actionable as a defamation, no matter how offensive, vituperative, or unreasonable it may be" and "[a]ccusations of racism and prejudice" have routinely been found to constitute non-actionable expressions of opinion); Williams v. Kanemaru (Haw. Ct. App. 2013) (accusation of racism based on disclosed facts not actionable for defamation); Lennon v. Cuyahoga County Juvenile Court (Ohio Ct. App. 2006) ("appellant's being called a racist was a matter of one employee's opinion and thus is constitutionally protected speech, not subject to a defamation claim").

Likewise, it's not actionable to accuse another of expressing sympathy for Communism, defending Communism, or even being Communist in his ideology. See, e.g., McAndrew v. Scranton Republican Pub. Co. (Pa. 1950); Clark v. Allen (Pa. 1964):

It is a matter of widespread common knowledge that countless patriotic Americans sincerely and sharply disagree as to what actions and/or words and/or policies aid the Communist cause, or what show Communist tendencies, or what amounts to an "appeasement" of Communism, or what is a "pro-Communist," or exactly what is meant by the term "soft on Communism."

While these words and expressions have a different meaning or meanings for very many Americans and often are undoubtedly intended to be derogatory, they are not libelous…. To hold these words or any of said expressions libelous would realistically and practically put an effective stop to searching and illuminating discussion and debate with likely dire results.

And to quote the general rationale for this position, from Judge Frank Easterbrook writing in Stevens v. Tillman (7th Cir. 1983):

The word ["racism"] has been watered down by overuse, becoming common coin in political discourse. Tillman called Stevens a racist; Stevens issued a press release calling Tillman a "racist" and her supporters "bigots." Formerly a "racist" was a believer in the superiority of one's own race, often a supporter of slavery or segregation, or a fomenter of hatred among the races…. Politicians sometimes use the term much more loosely, as referring to anyone (not of the speaker's race) who opposes the speaker's political goals—on the "rationale" that the speaker espouses only what is good for the jurisdiction (or the audience), and since one's opponents have no cause to oppose what is beneficial, their opposition must be based on race….

The term has acquired intermediate meanings too. The speaker may use "she is a racist" to mean "she is condescending to me, which must be because of my race because there is no other reason to condescend"—a reaction that attaches racial connotations to what may be an inflated opinion of one's self — or to mean "she thinks all black mothers are on welfare, which is stereotypical." Meanings of this sort fit comfortably within the immunity for name-calling.

Language is subject to levelling forces. When a word acquires a strong meaning it becomes useful in rhetoric. A single word conveys a powerful image. When plantation owners held blacks in chattel slavery, when 100 years later governors declared "segregation now, segregation forever," everyone knew what a "racist" was. The strength of the image invites use.

To obtain emotional impact, orators employed the term without the strong justification, shading its meaning just a little. So long as any part of the old meaning lingers, there is a tendency to invoke the word for its impact rather than to convey a precise meaning. We may regret that the language is losing the meaning of a word, especially when there is no ready substitute. But we serve in a court of law rather than of language and cannot insist that speakers cling to older meanings.

In daily life "racist" is hurled about so indiscriminately that it is no more than a verbal slap in the face; the target can slap back (as Stevens did). It is not actionable unless it implies the existence of undisclosed, defamatory facts, and Stevens has not relied on any such implication.

Now falsely accusing someone of a specific act—e.g., of firing or prosecuting someone because of the target's race—may well be libelous. Thus, for instance, in MacElree v. Phila. Newspapers, Inc. (Pa. 1996), the court held that characterizing the plaintiff as having acted improperly by "abusing his power as the district attorney, an elected office, to further racism and his own political aspirations" could be actionable. But the court specifically stressed that the statement did more than "merely label[ the plaintiff] a racist"—the statement was actionable because it focused on what the plaintiff supposedly did rather than just on what he believed, and thus "amount[ed] to a charge of misconduct in office."

So saying "Kyle Rittenhouse is a white supremacist" or "Saule Omarova is a Communist" (or "Socialist") isn't libelous, because that is understood as an opinion. But falsely asserting that "Kyle Rittenhouse had joined the KKK" or "Saule Omarova is a member of the Communist Party USA" may be libelous (at least unless the context shows that this is hyperbole or a joke or some such).