The United States was involved in a dispute with a small island nation over the ownership of an archipelago. On discovering that the archipelago was rich in oil, the President announced that he would appoint an ambassador to negotiate a treaty with the island nation to jointly exploit the oil reserve. A majority of Senators believed that the island clearly belonged to the United States and did not want to negotiate with the island nation. They passed a resolution requiring the President to include a Senator in his diplomatic mission to ensure that the Senate's view was presented in any negotiation with the island nation.

What is the strongest constitutional ground for the President's refusal to do so?