US President Barack Obama has earned sufficient congressional backing to ensure the international deal to
roll back Iran's nuclear programme goes into effect.
Barbara Mikulski of Maryland became the 34th senator to announce her support for the accord on Wednesday, which means any attempt by the US Congress to halt the deal will fail.
The agreement would ease punishing economic sanctions on Tehran while preventing Iran from advancing its nuclear programme.
Many Republicans claim that Iran will seek to cheat its way to an atomic bomb, Tehran denies any intention of attempting to produce such a weapon.
Should Congress pass a resolution that disapproves of the deal, Obama would veto it.
Overcoming the veto would require a two-thirds vote in both the Senate and the House of Representatives.
With Mikulski's support, the deal now has 34 backers in the Senate, the number needed to uphold such a veto.
Mikulski said on Wednesday that while the deal is not perfect: "I have concluded that this Joint Comprehensive Plan of Action is the best option available to block Iran from having a nuclear bomb."
The senator added that while she will vote for the deal, "Congress must also reaffirm our commitment to the safety and security of Israel", the AFP news agency reported.
Republicans are unified in their opposition to the deal, while two top Democratic senators, Chuck Schumer and Robert Menendez, are also opposed, as are a handful of Democrats in the House, including Steve Israel, the chamber's highest-ranking Jewish member.
Congress is set to vote on the accord later this month.