On Wednesday, Democratic Senator Barbara Mikulski became the last senator the administration of President Barack Obama needed to sustain the accord, against which Republicans had launched a campaign backed by the United States’ pro-Israeli lobbies.
Most US lawmakers are opposed to the deal, which would ease punishing economic sanctions on Tehran while preventing it from advancing its nuclear program.
But with Senate Democrat Barbara Mikulski announcing her support Wednesday, the deal now has 34 backers in the Senate -- the number needed to uphold a certain Obama veto should Congress pass a resolution that disapproves of the deal.
Overcoming the veto would require a two-thirds vote in the Senate and House of Representatives.
While the deal is not perfect, Mikulski said, "I have concluded that this is the best option available to block Iran from having a nuclear bomb."
Some Republicans have urged walking away from the deal and renegotiating tougher terms with Tehran, but US negotiators have warned that such a move could see the fragile international coalition that secured the agreement fall apart.
"It's unclear if the European Union, Russia, China, India and others would continue sanctions if Congress rejects this deal," said Mikulski, the longest-serving female senator in history. "Iran deal done. With Mikulski, Obama has all the votes he needs," number two Senate Republican John Cornyn posted on Twitter.