US secretary of state John Kerry and Treasury secretary Jack Lew testify before Tuesday's House foreign affairs committee hearing on the Iran nuclear agreement

Three US cabinet secretaries returned to Capitol Hill on Tuesday to persuade sceptical lawmakers to back a nuclear pact with Iran, as the White House seeks support for the deal.

The lobbying battle, which has pitted Barack Obama against vocal opposition from Republicans, the Israeli government and some well-funded pro-Israel pressure groups, has intensified, with Congress set for a make-or-break vote on the deal in September after its summer recess.

The president will meet House Democrats on Wednesday as he continues his push to sell the deal to his own party as well as to political opponents. Lawmakers are also receiving extensive briefings on the accord.

“I understand the fear, I understand the concerns that our friends in Israel have,” John Kerry, secretary of state and the lead US negotiator on the deal, testified in front of the House foreign affairs committee.

“But we believe that what we have laid out here is a way of making Israel and the region in fact safer.”

The deal, under which Iran would freeze elements of its nuclear programme and eliminate others in exchange for an easing of crippling economic sanctions, is a high-stakes attempt by Mr Obama to seal a lasting foreign policy legacy by curbing Iran’s nuclear ambitions, potentially for decades.

But even some of the pact’s staunchest supporters acknowledge that it contains uncertainties, with critics complaining about issues such as how quickly inspectors will be able to access suspected nuclear sites, how the US and its international partners will react if Iran cheats on the deal and whether the removal of sanctions will allow money to flow out of the country and into the coffers of terrorist groups.

Aipac, a pro-Israel group, dispatched hundreds of its members to the hill on Tuesday to lobby lawmakers against the deal, which Benjamin Netanyahu, the Israeli prime ministers, has called a “historic mistake.”

In a sign of how heated the debate has become in Washington, Republican presidential candidate Mike Huckabee attracted international scorn after he suggested over the weekend that the deal was akin to marching Israelis “to the door of the oven”, in a reference to the Holocaust.

Defending the deal on his first trip to Ethiopia, Mr Obama characterised the former Arkansas governor and talk show host’s comments as “ridiculous.

“The particular comments of Mr Huckabee are, I think, part of just a general pattern that we’ve seen that is, would be, considered ridiculous if it weren’t so sad,” Mr Obama said.

Mr Kerry, appearing with Jack Lew, the Treasury Secretary, and Ernest Moniz, energy secretary, continued the public relations offensive on Tuesday, telling the House foreign affairs committee that the deal, negotiated alongside France, Germany, Russia, the UK and China, represented the best chance of securing lasting security in the region.

“We have two options,” Mr Kerry warned. "Either we move ahead with this agreement to ensure Iran’s nuclear programme is limited, rigorously scrutinised and wholly peaceful, or we have no agreement at all — no inspections, no restraints, no sanctions, no knowledge of what they’re doing, and they start to enrich.”

Mr Obama has already vowed to veto any effort by Congress to block the deal.

Given widespread Republican opposition — particularly with the accord already a hot topic on the campaign trail for presidential candidates and lawmakers up for re-election in 2016 — the administration is battling to shore up enough Democratic support to ensure a Republican-controlled Congress cannot secure the necessary two-thirds majority in the House and Senate to overcome a presidential veto.

Polling on the deal has been uneven, with some surveys suggesting that most voters support the pact, with others showing the majority of Americans want Congress to block the agreement.

The latest poll, released by CNN on Tuesday, mirrored the sharp partisan divide on Capitol Hill, with 66 per cent of Republicans and 55 per cent of independents saying lawmakers ought to reject it, while 61 per cent of Democrats said the deal should be approved.

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