Democratic U.S. Senator Barbara Mikulski said on Wednesday she will support the Iran nuclear deal, giving President Barack Obama the 34 Senate votes needed to sustain a veto of any congressional resolution disapproving the deal.
Thirty-two Senate Democrats and two independents who vote with the Democrats now back the agreement.
"No deal is perfect, especially one negotiated with the Iranian regime. I have concluded that this Joint Comprehensive Plan of Action is the best option available to block Iran from having a nuclear bomb," Mikulski, of Maryland, said in a statement.
"For these reasons, I will vote in favor of this deal."
Mikulski's backing means Obama's fellow Democrats will have enough votes to protect the nuclear deal between Iran and world powers in the U.S. Congress. The agreement, announced on July 14, exchanges sanctions relief for Iran for Tehran's agreeing to curtail its nuclear program.
U.S. lawmakers have until Sept. 17 to vote on a "resolution of disapproval," which would weaken the international pact by eliminating Obama's ability to temporarily waive many U.S. sanctions on Iran.
With Republicans virtually united in opposition, Democrats have spent the summer rallying support for an agreement seen as a potential legacy foreign policy achievement for the president.
Republicans hold majorities in both the House of Representatives and Senate, and are likely to pass a Republican-sponsored disapproval resolution. However, Obama promised to veto any such measure, and opponents would have needed two-thirds majorities in both chambers to override the veto.
Support by 34 senators means a veto would be sustained.
A resolution would also fail if deal supporters can muster 41 votes in the Senate to block it using a procedural motion. Senate leadership aides on both sides of the issue said it was still too early to say whether that would happen.
Mikulski's support leaves 10 undecided Senate Democrats.