"There are substantial limitations which will help prevent Iran from building a nuclear weapon," Obama argued in a late-night appearance at the White House after the deal was sealed in Geneva. "Simply put, they cut off Iran's most likely paths to a bomb."
Some of the initial reaction from some members of Congress reflected a willingness to take a look at the agreement after weeks of criticism from lawmakers and key US allies Israel and Saudi Arabia.
Influential Republican Senator Lindsey Graham said on CNN that Congress would likely hold off on new sanctions for six months if Iran sticks to its part of the deal. Senate Majority Leader Harry Reid, an Obama ally, said in recent days that the Senate would pursue new sanctions.
"I think you'll see the Congress impose additional sanctions, it won't take place for six months with some conditions. If Iran meets certain conditions they will never go into effect at all," Graham said.
Democrat Representative Eliot Engel, the top Democratic on House of Representatives Foreign Affairs Committee, expressed concern about the deal. He was a lead co-sponsor of the new Iran sanctions act that passed the House on July 31 and has not yet been taken up in the Senate.
"While I am concerned that this interim agreement does not require Iran to completely halt its enrichment efforts or dismantle its centrifuges, I hope that over the next six months, Iran takes the necessary steps to finally end its quest for a nuclear weapons capability," he said.
Republican Representative Ed Royce, chairman of the House Foreign Affairs Committee, said he had serious concerns that the agreement does not meet the standards necessary to protect the United States.
"Instead of rolling back Iran's program, Tehran would be able to keep the key elements of its nuclear weapons-making capability," he said.
There was also outright scorn.
Republican Senator Mark Kirk, a member of the Senate Banking Committee, said he shared Obama's goal of finding a diplomatic solution to prevent Iran from acquiring a nuclear weapons capability but felt the terms of the deal were too lenient.
"This deal appears to provide the world's leading state sponsor of terrorism with billions of dollars in exchange for cosmetic concessions that neither fully freeze nor significantly roll back its nuclear infrastructure," he said.
He referred to a portion of the agreement that provides limited, temporary and targeted sanctions relief to Iran.
Florida Republican Senator Marco Rubio said the deal "shows other rogue states that wish to go nuclear that you can obfuscate, cheat and lie for a decade, and eventually the United States will tire and drop key demands."
Obama has the authority to waive sanctions for a period of several months and thus has the ability to sidestep congressional concerns but could risk a fight with Congress if he did so.
He and senior administration officials argued that the agreement was only the first step toward a deal to completely contain Iran's nuclear program. Six more months of negotiation lie ahead, they insisted.
"Going forward, we will continue to work closely with Congress. However, now is not the time to move forward on new sanctions - doing so would derail this promising first step, alienate us from our allies, and risk unraveling the coalition that enabled our sanctions to be enforced in the first place," Obama said.
The deal would represent a major foreign policy achievement for Obama, whose presidency has been hobbled in recent weeks by the troubled rollout of his signature healthcare law. His job approval ratings are hovering around 40 percent, the lowest of his less than five years in office.
Obama and his predecessor, George W. Bush, engaged in repeated attempts to persuade Iran to give up its nuclear weapons ambition, which it denies having, but failed each time.
Nurturing the latest attempt was an exchange of letters and a phone call in September between Obama and relatively moderate Iranian President Hassan Rouhani, the highest-level contact between the two countries in three decades.
Obama, seeking to reassure critics, said that if Iran did not meet its commitments during a six-month period, the United States would turn off sanctions relief and "ratchet up the pressure."