It was an unprecedented intervention which sought to undermine two-and-a-half years of diplomatic negotiations to strike a deal with Iran to curb its nuclear programme. Then Vice President Joe Biden slammed the "dangerous" intrusion.
"In thirty-six years in the United States Senate, I cannot recall another instance in which Senators wrote directly to advise another country - much less a longtime foreign adversary - that the president does not have the constitutional authority to reach a meaningful understanding with them," he said.
From the archives: Donald Trump doesn't understand the art of the Iran deal
The open letter was addressed to "leaders of the Islamic Republic of Iran" and warned that any deal without legislative approval could be reversed by the next president "with the stroke of a pen".
Drafted by Senator Tom Cotton, and signed by all but seven members of the Senate Republican majority, it came with a condescending lesson on the US constitution.
"It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system," said the letter.
It added: "We will consider any agreement regarding your nuclear-weapons program that is not approved by the Congress as nothing more than an executive agreement between President Obama and Ayatollah Khamenei. The next president could revoke such an executive agreement with the stroke of a pen and future Congresses could modify the terms of the agreement at any time."