A dictatorship, rather than oligarchy, is at least the implication of what has been outright said.
The U.S. Constitution was supposed to prevent one person getting all of the power. But there were some enormous loopholes, not least because of a lot of things the Framers never actually wrote into it, but just assumed that the U.S.A. would continue to do in the tradition of English common law.
Experts in #ConstititionalLaw knew of the problems in the 19th century.