President Obama has accused Republicans of conspiring with Iranian hardliners after they sent a letter to them saying that any nuclear deal with the US could be scrapped by the next president.
Forty-seven Senate Republicans, including several potential 2016 presidential candidates, sent the letter publicly addressing leaders of the Islamic Republic in a bid to scupper the sensitive talks.
The unprecedented move, attempting to undermine the president provoked Mr Obama to issue a stiff dressing down.
Joe Biden, the vice-president, who has decades of experience in Congress, concluded that the Republicans had put the US in peril with their misstep.
“It has come to our attention while observing your nuclear negotiations with our government that you may not fully understand our constitutional system,” the letter said.
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Republicans warned that any deal agreed before Mr Obama leaves office in 2017 is “nothing more than an executive agreement between President Barack Obama and [supreme leader] 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,” they added.
Mr Obama reacted by saying he would make his case to voters.
“I think it is somewhat ironic to see some members of Congress wanting to make common cause with hardliners in Iran,” he said. “It is an unusual coalition.”
“What we are going to focus on right now is actually seeing whether we can get a deal,” added Mr Obama.
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“Once we do - if we do - then we’ll be able to make the case to the American people.”
Mr Biden said: “This letter, in the guise of a constitutional lesson, ignores two centuries of precedent and threatens to undermine the ability of any future American president, whether Democrat or Republican, to negotiate with other nations on behalf of the United States.
“This letter sends a highly misleading signal to friend and foe alike that that our commander-in-chief cannot deliver on America’s commitments - a message that is as false as it is dangerous.
“Honorable people can disagree over policy. But this is no way to make America safer or stronger.”
With a March deadline looming, negotiators are working furiously to agree a deal that would curb Iran’s nuclear programme in return for reducing Western sanctions.
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A new round of talks between John Kerry, the US Secretary of State, and Mohammad Javad Zarif, the Iranian foreign minister, is due to take place in Lausanne, Switzerland on March 15.
The deal is seen as a key foreign policy goal of the Obama administration.
Many Republicans fear such an accord would loosen economic sanctions on Tehran while leaving it free to secretly attempt to develop nuclear weapons technology.
Iran insists it is developing nuclear power for civilian purposes.