They also tried it in the American South as part of Jim Crow: "literacy" tests that were hard for anyone to pass, with grandfather clauses so that all of the illiterate whites could keep voting (in fact, that's where the term "grandfather clause" comes from).

The problem with the term "moron" is that it is too easily applied to mean, "people who I don't think should be allowed to vote, no matter how intelligent they might actually be."