Iran’s pragmatist government tried yesterday to sell its nuclear agreement with world powers to hardliners at home, just as a US Congressional leader promised to do “everything possible” to sink the deal.

With both Tehran and Washington facing stiff opposition to the accord, US Defence Secretary Ash Carter arrived in Saudi Arabia in the hope of reassuring leaders there who fear their arch-rival Iran will make major mischief in the region.

Last week’s agreement was a big success for both US President Barack Obama and Iranian President Hassan Rouhani. But both have to promote it to influential hardliners in countries that have been enemies for decades.

In Washington, the Republican speaker of the House of Representatives, John Boehner, was deeply sceptical. “Members of Congress will ask much tougher questions this afternoon when we meet with the president’s team, and because a bad deal threatens the security of the American people, we’re going to do everything possible to stop it,” Boehner said.

Secretary of State John Kerry, Treasury Secretary Jack Lew and Energy Secretary Ernest Moniz were holding briefings for lawmakers in the US Capitol House and Senate debates yesterday. Votes to approve or reject the nuclear agreement are expected in September.

A warm glow following the Vienna agreement – under which Iran accepted curbs on its nuclear programme in return for an easing of sanctions that have crippled its economy – is fading.

Ayatollah Ali Khamenei, Iran’s highest authority, told supporters on Saturday that US policies in the region were “180 degrees” opposed to Tehran’s, in a speech punctuated by chants of “Death to America” and “Death to Israel”.

The government that negotiated the deal also talked tough yesterday in an apparent attempt to blunt attacks from opponents, including in the powerful Republican Guards.

Abbas Araqchi, a deputy foreign minister, said Iran would do “anything” to help allies in the Middle East, underlining Tehran’s message that the deal will not change its anti-Western foreign policy.

Araqchi, Iran’s senior nuclear negotiator, also told a news conference that any attempt to re-impose sanctions after they expired in 10 years would breach the deal.

He was referring to a resolution endorsing the deal passed by the United Nations Security Council on Monday. This allows all UN sanctions to be re-imposed if Iran violates the agreement in the next 10 years.

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