Answer:
The constitution.
Step-by-step explanation:
The Article I, section 4 of the dictates the way our country should conduct our election.
It granted the congress the ability to create regulations regarding the election. ([This include every small details of the election including the way people could cast off their votes, if there's any voting fee or not, how to count the vote, etc) On top of the election, this part of the constitution also regulate how political parties could appoint their candidates within the primary.