Hours after major powers agreed a deal over Iran's nuclear program, the White House launched a campaign to stop sceptics at home and abroad from derailing the long-awaited accord.
The agreement, signed on Tuesday after 18 days of marathon talks in Vienna, aims to ensure Tehran cannot create a nuclear bomb in return for lifting biting sanctions that have crippled Iran's economy.
It was hailed by the US, the European Union, Iran and NATO - all of whom hope the deal will end decades of bad blood between the Middle East's major Shiite Muslim power and the West - but branded a "historic mistake" by Tehran's arch foe Israel.
US President Barack Obama said the accord meant "every pathway to a nuclear weapon is cut off".
"This deal offers an opportunity to move in a new direction. We should seize it," he said in an address to the nation.
Hours after the deal was signed in Vienna the US had already begun its diplomatic offensive in the UN, where its diplomats were readying a draft resolution setting out timelines.
The document, expected in the coming days, would also replace the existing framework of Security Council sanctions with the restrictions agreed during negotiations in Vienna, US Ambassador Samantha Power said.
In Washington, Obama faces a challenge from Republicans who control Congress, who have said they will reject the deal as it gives Tehran too much room to manoeuvre and does not safeguard American security interests.
House Speaker John Boehner, a Republican, said the agreement was "likely to fuel a nuclear arms race around the world".
Congress has 60 days to review the deal reached between Tehran and the US, Russia, China, Britain, France and Germany to end a more than 13-year standoff, but the president has vowed to veto any attempt to block it.
Obama will hold a press conference on Wednesday to convince Americans, allies and sceptics about the benefits of the deal.
Underscoring the tectonic shift in relations, Iranian state television broadcast Obama's statement live, only the second such occasion since the 1979 Islamic revolution.
Iranian President Hassan Rouhani said in his own address that "God has accepted the nation's prayers" and that the accord would lift "inhumane and tyrannical sanctions".
"Iran will never seek a nuclear weapon, with or without the implementation" of the Vienna deal, he added.
On the streets of Tehran, Iranians festooned their cars with balloons and danced on the street in celebration.
"I was thinking about leaving, but now I will stay to see what happens," said 42-year-old computer programmer Giti.
The deal limits Iran's nuclear activities for at least a decade and calls for stringent UN oversight, with world powers hoping this will make any dash to make an atomic bomb virtually impossible.
In return Iran will get sanctions relief, although the measures can "snap back" into place if there are any violations.
The international arms embargo against Iran will remain for five years with deliveries only possible with permission from the UN Security Council, diplomats said.
Tehran has also accepted allowing the UN nuclear watchdog tightly-controlled access to military bases, an Iranian official said.
Iran will slash by around two-thirds the number of centrifuges, which can make fuel for nuclear power - and also the core of a nuclear bomb - from around 19,000 to 6104.
Painful international sanctions that have cut the oil exports of OPEC's fifth-largest producer by a quarter and choked its economy will be lifted and billions of dollars in frozen assets unblocked.
The agreement is Obama's crowning foreign policy achievement in six years, and the fruit of Rouhani's bid since his election in 2013 to end Iran's isolation.
Even politicians from Obama's own Democratic party were keen to show their tough stance, with presidential contender Hillary Clinton vowing Tehran will never be able to acquire atomic weapons if she is elected to the White House.
"As president, I would use every tool in our arsenal to compel rigorous Iranian compliance," Clinton said in a statement.