Republicans in Congress stepped up their efforts on Monday to complicate nuclear negotiations with Iran, with 47 senators sending an open letter to the Iranian leadership warning that the next US president could undo any agreement with the “stroke of a pen”.

Signed by the large majority of Republican senators, the letter is the latest attempt by the party to exert influence over the nuclear negotiations with Iran, including last week’s invitation for Israeli prime minister Benjamin Netanyahu to speak to Congress.

The Republican senators were immediately accused by both the White House and Congressional Democrats of trying to undermine the Obama administration’s foreign policy.

The letter comes as Iran and major nations are getting closer to the broad outlines of an agreement that would lift sanctions on Tehran in return for clear limits on Iran’s nuclear programme.

The unusual intervention into an ongoing negotiation underlines the strong opposition among Republicans to the emerging agreement with Iran, which is significant given that a permanent lifting of sanctions will ultimately require the support of Congress. Such fears could make Iran more reluctant to sign an agreement.

However, the letter could be counter-productive because it has accentuated the increasingly partisan terms of the debate over the nuclear negotiations at a time when some Republicans had been trying to build bipartisan and vetoproof support in Congress for two new pieces of Iran legislation.

Authored by Tom Cotton, a first-term senator from Arkansas, the letter argues that any Iranian agreement with the Obama administration might not be durable because Congress would be opposed.

Addressed to “the leaders of the Islamic Republic of Iran”, it suggests that “you may not fully understand our constitutional system”. If any agreement with Iran were a treaty, “the Senate must ratify it by a two-thirds vote”. If it were an international agreement, “the next president could revoke such an executive agreement with the stroke of a pen”.

Responding to the publication of the letter, President Barack Obama said it was “somewhat ironic to see some members of Congress wanting to make common cause with the hardliners in Iran”, forging what he called an “unusual alliance”.

Leading Democrats accused the Republicans of trying to scupper any chance of agreeing a deal with Iran. “Let’s be very clear: Republicans are undermining our commander-in-chief while empowering the Ayatollahs,” said Harry Reid, the minority leader in the Senate. He called the letter “juvenile” and written “purely out of spite”.

Joe Biden, the vice-president who was a senator for 36 years, said the letter was “expressly designed to undercut a sitting president in the midst of sensitive international negotiations”, and that it was “beneath the dignity of an institution I revere”.

Dianne Feinstein, the California Democrat, said that “I am appalled at the latest step of 47 Republicans to blow up a major effort by our country and the world powers to negotiate” a deal with Iran.

Josh Earnest, White House spokesman, said that the letter was “the continuation of a partisan strategy to undermine the president’s ability to conduct foreign policy and advance our national security interests around the globe”.

Mohammad Javad Zarif, the Iranian foreign minister, mocked the senators, suggesting that the letter misunderstood the legal implications of an agreement. “It may enrich the knowledge of the authors to recognise that according to international law, Congress may not modify the terms of the agreement at any time as they claim, and if Congress adopts any measure to impede its implementation, it will have committed a material breach of US obligations,” he said.

The letter was also criticised for arguing that the Senate ratifies treaties. Jack Goldsmith, a former assistant attorney-general under George W Bush, said that consent by the Senate was only one part of the process of ratifying a treaty.

“It appears from the letter that the Senators do not understand our constitutional system or the power to make binding agreements,” he wrote. “In a letter purporting to teach a constitutional lesson, the error is embarrassing.”

The letter was not signed by Bob Corker, the Tennessee Republican who chairs the Senate foreign relations committee and who has been trying to build broad support for two bills — one that would impose new sanctions on Iran if the talks fail and another that gives Congress the chance to vote on any final deal. Mr Obama has said he will veto both bills if they pass.

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