“Even preferable to state laws requiring presidential candidates to prove their constitutional qualifications before being allowed on the ballot would be a federal constitutional amendment requiring the same of all states.”
I suspect an amendment wouldn’t be required, as this would qualify the already existing eligibility requirement, as would a definition of “natural born citizen”.
Congress wouldn’t be adding anything to the U.S.Constitution, just clarifying and expanding within the parameters of one of the eligibility requirements.
I beg to differ with that. There would be something added by such an amendment: a constitutional mandate to state governments to conduct elections for president (and possibly other federal offices) in a specific manner, whereas currently the Constitution (and federal law) is silent on this specific subject, and states have the discretion to check such candidates for constitutional qualifications or not to do so.
Of course, this would not be the first constitutional mandate to state governments, and I wouldn't be concerned about it being considered an abuse of "states' rights," because the states would actually be taking on more authority.