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."