I think you're right that this probably couldn't be done without an amendment, but I don't see how California Democratic Party vs. Jones actually is a strong ar

I think you're right that this probably couldn't be done without an amendment, but I don't see how California Democratic Party vs. Jones actually is a strong argument for that. That seems to largely have been a first amendment case, if I understand the opinion correctly. The basic argument being that blanked primary systems forced parties to associate with candidates, which'd in turn be forced speech ("freedom of association"). Isn't the issue more around Article 1 enumerated powers?