US President Barack Obama has said that partisanship over the Iran nuclear deal has gone too far.
He rebuked the stance of some Republicans in the US Congress.
Some Republicans have argued against the deal, saying that Iran has received too many concessions.
An outline agreement on the future shape of Iran’s nuclear programme was reached after marathon talks with six major powers earlier in April.
The deal aims to prevent Tehran making a nuclear weapon in exchange for phased sanction relief.
A deadline has been set for 30 June to reach a comprehensive pact. Tough negotiations still lie ahead.
President Obama, speaking after a regional conference in Panama, said he remained “absolutely positive” that the deal was the surest way to prevent Iran obtaining nuclear arms,
Earlier, Iran’s Supreme Leader Ayatollah Ali Khamenei said that a final agreement must result in an immediate end to all sanctions.
President Obama said on Saturday that Mr Khamenei was simply addressing his own country’s internal politics.
“Even a guy with the title ‘Supreme Leader’ has to be concerned about his own constituencies,” he said.
“There may be ways of structuring a final deal that satisfy their pride, their optics, their politics, but meet our core practical objectives.”
Mr Obama went on to criticise the attitudes of some Republican senators who have been highly sceptical about the emerging agreement with Iran.
Last week, Senator John McCain said that US Secretary of State John Kerry’s explanations of the agreement were less trustworthy than those of Iran’s supreme leader.
Mr Obama said that entrenched partisanship was no way to run foreign policy.
“I don’t understand why it is that everybody’s working so hard to anticipate failure,” he said.
The framework agreement was announced by the European Union and Iran after eight days of negotiations in Switzerland.
The talks at Lausanne’s Beau-Rivage Palace hotel between Iran and the so-called P5+1 – the US, UK, France, China and Russia plus Germany – continued beyond the original, self-imposed deadline of 31 March.
The outline agreement has been criticised by members of Congress who want US lawmakers to have the right to review any final deal.