One interesting aspect of the Brexit decision was that it involved a legally nonbinding referendum. The UK Parliament was not legally bound to follow the result, but nearly everyone accepts the result, with a statement like, the people have spoken and we have to follow it. I think part of the reason for this is that prior to the vote, it was recognized that this decision would be decisive, even though it was technically nonbinding. Thus, it would be morally illegitimate not to follow the decision because one did not like the result. Keith Whittington makes a similar point about the ratification conventions at the time of the Constitution.
Would it be constitutional for the United States to hold a similar nonbinding referendum? Let’s imagine that the U.S. was faced with a decision that would be consistent with the Constitution — say deciding whether to withdraw from NATO or the UN or the WTO. It is clear that some political actor — whether the Congress or the President or the President and the Senate, depending on the circumstances — would have the authority to make this decision. Could there be a nonbinding referendum held on the matter?
The principal issue is whether Congress would have an enumerated power to establish the referendum. I believe that it would. Let’s assume that the issue is whether to withdraw from NATO and that the President was the political entity with the authority to make the decision. Congress could then argue that it would be helpful for the President, in making his decision, to know what the American people thought of the issue, and therefore there should be a referendum. Congress could then argue that this referendum was necessary and proper to carrying into execution the President’s authority under the Constitution.
This seems like the correct constitutional argument here. It is analogous to Congress deciding to authorize a public opinion poll on the same issue.
Of course, if Congress were authorizing the referendum (or the public opinion poll) in order to delegate the decision to the American people, then one might argue it was not necessary and proper to an enumerated power. But so long as Congress argues that it is simply acquiring the information to help the President make his decision, then the referendum would seem to be constitutional.