US Senator Mazie Hirono became the 21st Democratic senator to support the Iran nuclear deal Monday, calling it “the best option to halt Iran’s nuclear weapon program.”

Hirono, from Hawaii, said she based her decision on what she saw as the efficacy of the deal in stopping Iran’s nuclear weapon program, the retained ability of the international community to use sanctions or military action in case of a breach, and international opposition to negotiating a new deal.

Her statement came as Senate Leader Mitch MCConnell admitted that the Iran nuclear deal would likely not be stopped, with critics unable to muster the needed 67 “no” votes to override an expected presidential veto.

Hirono said that the benefits of the deal outweighed the risks and that with the passing of the deal “Iran’s nuclear weapons program, which was on the verge of success, will be disabled for many years.”

“While this agreement is not perfect, it has gained broad national and international support, including 29 top American nuclear scientists, of which six are Nobel laureates. This agreement is the best option to halt Iran’s nuclear weapon program,” Hirono added.

She also said that Chinese, British and Russian ambassadors had told her their countries would not be open to resuming negotiations should the US reject the agreement that seeks to curb Iranian nuclear activity, brokered in July between Tehran and six world powers after years of torturous negotiations.

“They unanimously said, ‘No,’ that there was already a deal– the one before Congress,” said Hirono.

Hirono added that she shared the concerns over Iran’s support for terrorism and that she would “support increased efforts, along with our allies, to counter Iran’s destabilizing and malign actions within and outside the Middle East.”

So far 21 Democrat senators have come out in favor of the deal, including Hawaii Senator Brian Schatz, who is Jewish. Schatz said last week that there was “no other alternative that achieves these results.”

So far New York Senator Chuck Schumer is the only Democrat to say he will vote against it.

In addition, 54 House Democrats have voiced their intention to support the deal in its upcoming vote.

Congress began a 60-day review of the nuclear agreement last month, with a vote expected in September.