US president Barack Obama has secured a landmark foreign policy victory amid opposition from Republicans and the government of Israel.
The win was achieved when Democratic senator Barbara Mikulski became the 34th vote in favour of the Iran nuclear deal.
Ms Mikulski's backing gives supporters the margin they need to uphold an expected Obama veto of a congressional resolution of disapproval that Republicans hoped to pass later this month.
And it spells failure for opponents of the international agreement who sought to foil it by turning Congress against it. Leading that effort were Israel and its allies in the US, who failed to get traction after spending millions of dollars trying.
The agreement signed by Iran, the US and five other world powers limits Iran's nuclear programme in exchange for relief from hundreds of billions of dollars in sanctions. Republicans and Israeli leaders contend that concessions made to Iran could empower the country, which has sworn to destroy Israel.
"No deal is perfect, especially one negotiated with the Iranian regime," Ms Mikulski said in a statement. "I have concluded that this joint comprehensive plan of action is the best option available to block Iran from having a nuclear bomb. For these reasons, I will vote in favor of this deal."
Secretary of state John Kerry is sending a letter to all members of Congress outlining US security commitments to Israel and the Gulf Arab states in light of the nuclear deal.
The letter comes as Mr Kerry delivers a major policy speech in Philadelphia that focuses on how the international agreement makes the US and its allies safer.
"I really believe the fastest way to a genuine arms race in the Middle East is to not have this agreement," Mr Kerry said. "Because if you don't have this agreement, Iran has already made clear what its direction is."
With opposition to the agreement failing to take hold on the Democratic side, supporters may even be able to muster the 41 votes needed to block the resolution from passing in the first place, sparing Mr Obama from having to use his veto. That would require seven of the 11 remaining undeclared senators to decide in favour of the deal.
Only two Democratic senators have come out against the deal - Chuck Schumer and Robert Menendez - while in recent weeks undeclared Democratic senators, even from red states, have broken in favour one after another.
Even if Congress was able to pass the disapproval resolution, it can not stop the deal, which was agreed to among Iran, the United States, Britain, France, Germany, Russia and China. In July, the UN Security Council unanimously endorsed the nuclear deal, approving a resolution that would lift the international sanctions on Iran in 90 days.