Post by Olivier
Good morning, sir.
I heard that all US presidents have the blood of English kings.
This is statistically likely, but not genealogically accurate. Periodically this kind of claim gets reported in the popular media, but the actual historical record does not bear it out. The pedigree of Andrew Jackson, for example, can't be traced beyond a great-grandparent, and there are others who have pretty obscure backgrounds that can't be traced before the 16th century. The people making the claim are basing it on sloppy genealogy.
There are variants of this claim that have been broadly reported. One was by an amateur American genealogist who purchased the publishing rights to one of the British Peerage trademarks and then portrayed himself as an expert on all things genealogical. Every four years for about two decades, he would send out a press release to the media claiming that in every American election, the candidate with the most/best royal ancestry won, and this inviolable rule could be used to pick the winner of the next election. It would be reported in the nature of the stories about the octopus who picked the winner of the World cup, or more traditionally, the groundhog who predicted then spring would arrive, but some people took it as serious reporting. Even if he picked it wrong, he would be back again the next election cycle claiming to have discovered a 'better' royal descent for the actual winning candidate than he knew about before the previous election, and thereby confirming his principle. He never provided any details on the pedigrees involved, and there is every reason to believe he was simply a charlatan, making it all up to get media attention. (In his obituary in a British newspaper, it was said that his upside was that he was always available to the press with a pithy fact they could publish, the downside being that it was usually incorrect.)
As an aside, his claim would be picked up by David Icke, a conspiracy theorist who believes that the Illuminati who have been secretly controlling the world since prehistoric times and from whom all of the royal houses are drawn, and because of this work, all of the presidents as well, are actually the descendants of alien reptilian humanoids.
The other version was by a 12-year-old who simply went on line to the crowdsourced pedigrees on FamilySearch and 'discovered' that every American president descended from king John. The media reported it as a 'look what the precocious girl has done' story, without fact-checking the original claim. Well, the FamilySearch trees are of similar quality to other crowdsourced genealogy collections that are online - a few carefully done, using appropriate scholarship with primary records, but most are the work of simple name-collectors, compiling their trees from other trees they in turn found online, without any understanding of the sources, caveats or anything else, and thus combining every jumped conclusion, fanciful assumption, etc. Again, there are a number of American presidents for whom no such authentic line has been traced.
Post by Olivier
I'd like to know if the candidates for 2020 have that particularity.
I don't think Trump has such a documented line (if he did, I am sure NEHGS would have issued a press release by now, reporting "Trump is a cousin of Obama"). I seriously doubt Bernie Sanders does (his background is continental Jewish), and I am completely unfamiliar with Biden's roots.