WASHINGTON — President Obama wants any nuclear deal reached with Iran to last at least 10 years. But a threat by Republican senators raises the question of whether it would last even 22 months.

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.

“He’d be violating an international agreement with another country, but the president has the power to do that,” 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 “it’s sort of like judicial positions or positions in briefs that a prior administration has taken. There’s sort of respect for precedent.”