The problem with saying that Warren G. Harding was the first African-American President is that it is a theory that is unproven by fact.  While many people believe this concept - some even go to great lengths to self publish books on the topics, there is no way to scientifically prove the concept, nor is there any way to verify generally accepted documents that could show this is even a possibility.  The rumor that Harding was black rages about based on two quotes, one by Harding himself (where he says he doesn't know if it could be true, the other a rumor started by his future father-in-law meant to destroy Harding's business and reputation.) Consider verifiable these FACTS on the matter:
1) Harding's alleged "black" lineage can not be proven through United States Census forms, death certificate, or any other PRIMARY SOURCE document.  As far back as the 1850 census (the first to name all people enumerated, and designate their race) Harding's father and mother are enumerated as white.  This is most frequently dismissed by Harding Conspiracy followers who claim the census forms were doctored after the fact as part of a Federal Government cover-up.

2) There is no DNA proof of Harding's alleged "black blood", (nor is there proof that Harding fathered Elizabeth Ann Christian).

3) Harding was not raised as a cultural "black" that is to say that he was not raised within the black community, and therefore would have not had the same cultural experiences that other blacks in the era would have had.

4) "Black Harding" promoters will point to William Estabrook Chancellor's book on Harding as proof of government conspiracy.  And it is interesting how swifty the government worked to squash the book and get its copies.  But if one looks at Chancellor's research methodology (and we step away from those who claim that the book was supressed) - all of it based on hearsay - any student in college today who would present this type of research in the form of a paper, masters thesis or dissertation would have been thrown out of college for faulty research!   

5) Some of the promoters of this theory of Harding's "blackness" on "family stories".  While folklore is not always untrue, folklore can be embellished from storyteller to storyteller. Ask anyone who studies it, folklore is at the mercy of the one who passes it on.

6) The promoters of these theories go to great lengths to attack the people who poke logical holes in their wishful arguments. They can not refute the facts, so they attack the people who question their beliefs. Take a look at the "reviews" of Harding books published in the past ten years in places like Amazon and B&N; online and you'll get an idea how some people will attack those who do not buy into the black "Black Harding" myth.

If Wikipedia were to add information that Harding was the first black President, without irrfetable proof, it simply ads another chain in the long list of unrliable misinformation.