President Barack Obama says he has yet to hear a strong factual argument "that holds up to scrutiny" against the nuclear deal with Iran.
Speaking at a news conference in Addis Ababa, Ethiopia, Obama criticized rhetoric about the agreement from some leading Republicans.
"The reason why 99 percent of the world thinks it is a good deal is because it is a good deal," he said.
The Iran deal has been approved by the U.N. Security Council, but despite lobbying and testimony by top Obama administration officials, it has won few new congressional supporters.
If the deal is rejected, Obama has promised to veto that decision.
To override the veto, it would take a two-thirds majority in both chambers of Congress, meaning many members of the president's Democratic Party would have to join Republicans in rejecting the deal.
Last week, top administration officials faced tough Senate questioning in the first public hearing on the deal, which Congress has to accept or reject by mid-September.
Secretary of State John Kerry, appearing as a witness, was told by Foreign Relations Committee Chairman Bob Corker, "you've been fleeced" by the Iranians. Kerry responded by saying "they already have what they want ... it is a question of how do you hold their program back."