In their protocol-breaking letter to Iran this week, nearly four dozen Senate Republicans suggested that the next president could simply reverse such an agreement after Mr. Obama leaves office in January 2017. On one level, they were right. A new president could decide to not go along with the terms inherited from his predecessor.
But it would be an extraordinary breach of tradition, one that most presidents have avoided. As a practical matter, presidents generally do not break international agreements because it could call into question the reliability of other agreements, alienate allies and set a precedent that few occupants of the White House want to set since they would like their pacts honored after they leave office.
âHeâd be violating an international agreement with another country, but the president has the power to do that,â said John B. Bellinger III, who served as the chief lawyer at the National Security Council and later the State Department under President George W. Bush. Few do so, he added, because âitâs sort of like judicial positions or positions in briefs that a prior administration has taken. Thereâs sort of respect for precedent.â
Graphic: The Nuclear Talks With Iran, Explained
The letter, drafted by Senator Tom Cotton of Arkansas in response to concerns that Mr. Obamaâs deal would not do enough to prevent Iran from building nuclear weapons, touched off a sharp debate about the boundaries between the executive and legislative branches on foreign policy. Even as Republicans emphasized the limits of a deal brokered unilaterally by a president without submitting it to Congress, the White House and its allies fumed.
The letter seemed aimed at sowing doubt among Iranian leaders that the United States would live up to the terms Mr. Obama is offering and therefore perhaps scuttle the talks. At first, Iran brushed off the threat, with its foreign minister dismissing it as a âpropaganda ployâ with no legal impact.
But Hamid Abutalebi, a key adviser to President Hassan Rouhani, issued a statement warning that congressional interference should be taken seriously. âThis move must not be easily ignored,â he said.
Ellie Geranmayeh, an Iran expert at the European Council on Foreign Relations, said the letter bolstered Iranâs supreme leader, Ayatollah Ali Khamenei. âEven if the talks fail because Iran takes a maximalist position when negotiating behind closed doors, the blame will nevertheless be placed on the U.S. legislators for poisoning negotiations,â she said.
The debate underscored how modern presidents have exerted their executive authority in forging deals with foreign countries, much as they have expanded their authority to make war. While the Constitution requires a two-thirds Senate vote before a treaty can be ratified, presidents often skirt this requirement by negotiating what are called executive agreements with foreign counterparts.
Since the 1930s, in fact, 94 percent of pacts between the United States and other countries have been executive agreements and just 6 percent have been treaties submitted to the Senate, according to a study by Glen Krutz of the University of Oklahoma and Jeffrey Peake of Clemson University. The vast bulk of them involve minor matters, like rules and practices in an increasingly globalized economy. But sometimes they involve controversial issues.
In 2008, for instance, Mr. Bush signed an agreement with Iraq permitting American forces to remain there until the end of 2011. At the time, Mr. Obama, then a senator, and other Democrats objected that Congress should get to vote on it. âThe notion that President Bush could somehow tie the hands of the next president, I think, is contrary to how our democracyâs supposed to work,â Mr. Obama said then.
Now, on the issue of whether Congress should be consulted on such agreements, the positions have flipped: Republicans who supported Mr. Bushâs authority to make his agreement are trying to insert themselves into Mr. Obamaâs dealings with Iran, while Mr. Obama is asserting his power as president to act on his own.
The letter warned that any nuclear agreement could be reversed by the next president âwith the stroke of a pen.â
In the Iran case, Mr. Obama would lift sanctions he has imposed under executive authority if Tehran agrees to curb its nuclear program for at least 10 years. He also has the power to suspend other sanctions imposed by Congress. Where lawmakers would have a role is if Mr. Obama agreed to completely lift congressional sanctions, which only Congress could do permanently.
A coalition of Republican and Democratic lawmakers has crafted legislation to force Mr. Obama to submit the agreement to Congress for a vote, but some of those same Democrats are angry at the Republican decision to release the letter to Iran, seeing it as partisan.
Other Democrats even suggested it may violate the Logan Act, a law passed under President John Adams making it illegal for any citizen âwithout authority of the United Statesâ to carry on a correspondence with a foreign government âwith intent to influence the measures or conductâ of that government or âto defeat the measures of the United States.â
While lawmakers in both parties have been accused of violating it over the years, it has never been enforced in modern times. Lawyers and former officials said the Republican letter probably would not cross the legal line because it was styled as an âopen letterâ and not actually sent to Iran, similar to an op-ed in the newspaper.
âThis is an open letter, an open statement, not an interference in negotiations,â said Elliott Abrams, a veteran of the Ronald Reagan and George W. Bush administrations. âThey have no vow of silence on foreign affairs.â
Still, it was up for debate whether a new president would unravel a deal set by Mr. Obama. Mr. Bush abrogated the Anti-Ballistic Missile treaty after taking office, but by exercising a withdrawal clause included in the document. He also renounced the Kyoto climate change treaty negotiated by Bill Clinton, but that treaty had never been submitted to the Senate for a vote and so was never in force.
Backing out of an Iran agreement would be more complicated, especially because it also would mean bailing out on allies like Britain, Germany and France. âTechnically, a president could change things,â Mr. Krutz said. âBut theyâre so unlikely to do so on a previous executive agreement because there are multiple other signatories.â
That would be even more complicated if the Iran agreement is taken to the United Nations Security Council for approval. âThe U.N. nature of it adds a level of legality that makes it different from just an agreement between the United States and Iran,â said Mark Fitzpatrick, an Iran expert at Londonâs International Institute for Strategic Studies. âA Security Council resolution will give it standing that makes it even more concrete and harder for a successor to simply undo.â
Indeed, a United Nations resolution would enshrine legal obligations for Iran, too, in terms of inspections and verification, said Camille Grand, director of the Foundation for Strategic Research in Paris. âThe Security Council is meant not to bless the deal,â he said, âbut carve it into marble.â
An earlier version of this article referred incorrectly to the director of the Foundation for Strategic Research. The director, Camille Grand, is a man.