The Republican Party is a private entity. Thus, it can set and establish any rules it wants.
For an individual or entity to bring an outside lawsuit would raise the question of what that person has any ‘standing’ [a legalese term] to bring such a lawsuit. Since the Party is a private organization, the bringer of the lawsuit would probably have no standing.
Look for this to be filed under ‘frivolous’.
This should and will be squashed just like the Arizona delegate tantrum, last week.