The "functionalist" tradition of flexibility in construing presidential power, especially in light of practical needs and congressional acquiescence, manifests itself perhaps most dramatically in the domains of war and foreign affairs.
The Executve has repeatedly claimed authority to act unilaterally in matters of foreign affairs, largely on the theory that the United States must be able to speak with a single decisive voice on the world stage. For the Most part, both courts and Congress have acceded to this claim. The Constitution provides that the President can negotiate treaties "by and with the Advice and Consent of the Senate...provided two thirds of the Senators present concur.
This prescribed process makes secret negotiations difficult; it also permits as little as one-third of the Senate to block a treaty. At the end of WWI, for example, a relatively small band of senators succeeded in blocking ratification of the Treaty of Versailles and in keeping the US out of the League of Nations. As an alternative to the treaty process, Presidents have subsequently claimed an authority to enter into, " executive agreements", with the same force of law as treaties, without seeking Senate approval. In important cases decided during the 1930s and 40s, the SCOTUS held that an executive agreement between the Roosevelt administration and the Soviet Union was legally valid and that it both created judicially enforceable federal rights and over-rode competing claims based on state law. By permitting an executive agreement to nullify otherwise applicable state law, the Court upheld a power of unilateral presidential lawmaking, the scope of which remains uncertain.
During the 1980s, the Supreme Court again held that the President could eliminate rights to sue a foreign government in American courts, this time under an executive agreement concluding a crisis that had involved the seizure of American hostages by the government of Iran. In Dames & Moore v Reagan (1981), The Court emphasized the need for executive flexibility in matters involving foreign relations. It was vital to get back the American hostages, desirable to get the deal done swiftly on terms acceptable to the Iranians, The Court also found implicit congressional authorization for the president to act unilaterally but acknowledged that no statute conferred the power directly. Today it is no longer clear when the President must seek Senate ratification of a treaty in order to conclude a legally binding agreement with a foreign government.
However the Court relies heavily on precedent and despite the howling of the members of the Senate, Obama doesn't need them to be involved in the negotiations with Iran. In fact, they have no authority to act as a separate government, which is what they're doing, or to speak for the government of the United States. They don't. The President does that. Each Senator was elected within his given state. Senator Rand Paul was elected to the Senate by the people of Kentucky, not the people of California or Illinois. He answers to the people of the state he represents. The President was elected in a General election by all the people of the United States. The voice that speaks for America is reserved to the office of the President. Like petulant children, they've announced to the leaders of Iran, that they shouldn't trust Obama or the United States to live up to the terms of the deal. Not only did they invite a foreign leader to come to American to speak to this country from the House of Representatives expressly to undermine the Presidents foreign policy, they've doubled-down on their stupidity and displayed their obvious hatred for the president by sending a letter to our enemies, telling them not to trust our president of the US in honoring the terms of the deal, and that they'll undo it at first opportunity. And in their ham-handed way, have condescended to the leaders of Iran with regard to our constitution and demonstrated that they know less about it than the Iranians. They've most certainly violated the Logan Act which has never really been applied before, but then, nobody has ever acted as stupidly as this group of Senators in a partisan display of disrespect for this president and the office itself.