US Secretary of State John Kerry says it would be embarrassing to him and a blow to US credibility on the world stage if Congress rejects the Iran nuclear deal.
"Do you think the ayatollah is going to come back to the table if Congress refuses this and negotiate again?" he told a well-heeled audience at the Council of Foreign Relations in New York on Friday.
Kerry has spent recent days trying to sell the hard-fought deal to sceptical Americans as the Republican-majority, 535-member Congress has 60 days to review the deal.
Congress can pass a motion of disapproval, which President Barack Obama can veto. An override of the veto requires two-thirds approval in both the House and Senate.
"Do you think that they're going to sit there and other people in the world are going to say, 'hey let's go negotiate with the United States, they have 535 secretaries of state'?," Kerry said.
"I mean please. I would be embarrassed to try to go out. What am I going to say to people after this as secretary of state. 'Come negotiate with us.' 'Oh Can you deliver?' Please."
Kerry faced blistering criticism on Thursday that he had been "fleeced" and "bamboozled" by Tehran, as he defended the Iran nuclear deal publicly on Capitol Hill.
He encountered a tsunami of scepticism from Republicans on the Senate Committee on Foreign Relations, during a marathon four-and-a-half-hour hearing.
The UN Security Council unanimously endorsed the agreement on Monday, paving the way to the lifting of punishing economic sanctions.
The deal was reached in Vienna last week after tough negotiations between Iran and the permanent Security Council members - Britain, China, France, Russia and the United States - plus Germany.