In return for lifting U.S., EU and U.N. sanctions, Iran will be subjected to long-term curbs on a nuclear program.
The battle may determine the fate of the president's signature foreign policy achievement.
To renege on the agreement would damage the U.S.'s credibility and its future capacity to act.
With the Greek and Iranian deals done, the hard work begins.
The deal will not produce an Iranian epiphany about the horrific consequences of helping Bashar Al Assad.
The president repeatedly stressed that the agreement has multiple safeguards to prevent Iran from attaining a nuclear weapon.
Iran has hanged more than 650 people already this year and is one of the world's largest prisons for journalists and bloggers.
"The strange inspection arrangement in the agreement will allow the Iranians to lie and cheat."
"We are working hard, but a deal tonight is simply logistically impossible."
Diplomats have until Monday to reach an agreement, their third extension in two weeks.
"We're here because we believe we are making real progress," Kerry said.
"We're frankly more concerned about the quality of the deal than we are about the clock," the spokeswoman for the U.S. delegation said.
Six major powers and Iran have decided to extend the original June 30 deadline to July 7 to allow more time for negotiations.
Diplomats said the talks in Vienna would run for as long as necessary to reach a deal.
The State Department accused Tehran of kidnapping and amputations on Thursday.
Alongside monitoring trade, the financial sector has a key role to play—if its rules are set right.
The Camp David summit is just the start in a long process needed to calm a troubled region.
Only two heads of state will attend as Gulf states seek reassurance over US ties with Iran.