There is no guarantee that a person be from a political party, there is nothing unconstitutional about this procedure.
Not only did the Founders not support political parties, they abhorred them. They were “factions” and always trouble in a republic.
Primaries were not part of the landscape for fifty years.
Political parties are a natural feature of government. The left is gonna organize so we got to organize to stop them, period.
Thinking they would not or should not exist was naive.