Since it’s clear that Congress won’t be rejecting the Iranian Nuclear Agreement, opponents are now trying to diminish its significance. Senator Marco Rubio, for example, now denies that the pact is “binding on the next administration,” and has pledged to repudiate it on his first day as president.

Rubio is wrong. If taken seriously, his position would destroy the binding character of America’s commitments to the IMF, the World Bank, NAFTA, and the World Trade Organization. The accords that undergird these institutions, like the Iran agreement, have their foundation in statutes authorizing the president to commit the nation. The Constitution makes these statutes the “supreme law of the land,” binding the country in the same manner as treaties approved by the Senate. The President can no more walk away from them than he can from any other law or treaty.

Congressionally authorized executive agreements have long served as the mainstay of American diplomacy. Since the Second World War, they have provided the basis for 90 percent of America’s international obligations.

Two factors have driven the dramatic ascendance of these agreements. Since only the Senate is involved in ratifying treaties, the House may balk when it is later asked to support the treaty with implementing legislation and funding—leaving the country in default of its legal obligations. In contrast, the statutory route requires a majority buy-in from both houses of Congress from the very beginning.

Treaty-making is also less democratic. California has a population equal to that of the 22 American states with the fewest inhabitants, yet it has only 2 senators and they have 44. Since ratification requires a two-thirds majority in the Senate, senators from sparsely populated states may readily veto presidential initiatives favored by the overwhelming majority of Americans. This deficit of democracy is diminished, though hardly eliminated, when a simple majority of the Senate joins the House in authorizing a diplomatic initiative.

The constitutional text, democratic principles, and entrenched practice overwhelmingly support the binding character of the Iran agreement. So does legislation that Congress adopted last May, which explicitly grants the Administration authority to negotiate and implement binding legal commitments with Iran.

Other provisions of the May legislation reinforce the high seriousness with which Congress has taken the president’s initiative. The statute, in setting down the procedural ground rules for negotiations, required the president to give the House and Senate roughly 60 days to reject the terms of any settlement he might reach with the Iranians.

This represents a sharp departure from standard practice. Generally speaking, Congress finds it too time-consuming to pass judgment on the individualized deals that the executive negotiates with foreign governments. It instead allows them to go into effect immediately so long as they comply with the rules set out in the authorizing statute. Congress insists on case-by-case review only on matters of fundamental importance—it singled out these negotiations for special attention precisely because they could create a long-term arrangement,  lasting 15 years, that  will govern the conduct of several presidents.

Now that Obama’s initiative seems to have survived this process of review, Rubio’s response should be seen for what it is: an unprecedented pledge to inaugurate his term by repudiating the constitutional command to “take care that the laws be faithfully executed.” There has already been too much lawlessness from the presidency since September 11th; Rubio’s celebratory anticipation of further illegalities represents a new low.

His loose rhetoric will also have broad-ranging implications, even if he fails to win the presidency. The Iran agreement isn’t the only hot-button negotiation approved by Congress. The administration is currently engaged in intensive efforts to win breakthrough Pacific Rim and European trade pacts, which are already slated for “fast-track” consideration by Congress.

The prospect of legislative review is already complicating negotiations, since foreign governments can’t be sure that the administration can can convince the House and Senate to approve  a bargain that satisfies their key demands. Nevertheless, momentum is building for a real deal, at least for the Transpacific Partnership.

But there’s one caveat. Up to now, foreign governments have been assuming that if Congress says yes, their deal is legally binding. Yet if Rubio’s assault on congressionally authorized agreements gains ground, the entire diplomatic effort will likely come to a halt: America’s bargaining partners will refuse to alienate domestic supporters by making big concessions once they’re told that the next president can abandon the entire arrangement.

The Constitution is bigger than any particular partisan dispute. If other opponents of the Iranian or Transpacific agreements join Rubio in assaulting the current framework governing American diplomacy, all they will accomplish is the destruction of the country’s credibility—undermining the ability of the United States to serve as a leading architect of world order in the 21st century.