> OTOH, you could argue that Constitutional amendments are national
Only if you want to change the meaning of the word “referendum” in such a way as to make it useless. A referendum is something voted on directly by the voters of a jurisdiction. It’s not something voted on by a legisture, regardless of how the legislature is elected. To pass an amendment to the U.S. constitution, first you have to propose the amendment by either
Having a bill get a two-thirds vote of both the Senate and the House of Representatives
Getting the majority of the vote at a convention elected specially for that purpose when two-thirds of the legislatures of the states call for it
and then it has to be confirmed by either
Getting a majority vote of three-quarters of the state legislatures
Getting a majority vote of three-quarters of the conventions elected specially for that purpose in each state (i.e., each state will have its own convention specially elected)
Now, it’s arguable that if a national convention was elected as in the second option of the proposal process or if state conventions were elected as in the second option of the confirming process, then the vote for the representatives might be close to a referendum, since they would be elected only for the purpose of voting on an amendment, which would mean the the representatives to such a convention would run purely on the basis of what their vote on the amendment would be. However, I believe that the national convention option has never been used and the state convention option has only been used once. If you use the word “referendum” for a state legisture voting on confirming an amendment, you might as well say that any bill in a state legislature is a referendum, since each member of the legislature was elected.