U.S. Sen. Mazie Hirono announced her support for the Iran nuclear deal Monday evening in her Honolulu district office, becoming the 21st senator to come out publicly for  the diplomatic agreement, which she called “the best option we have to halt Iran’s nuclear program.”

Hirono said she’s studied the accord for weeks, taking part in numerous classified briefings and public hearings and reading volumes on the matter. Over the past week, she has also listened to constituents in Hawaii, most of whom have said “it’s the best option for us, for peace,” said Hirono.

In the end, three key points were essential to securing her support, she said. First, without the deal, Iran has sufficient material to make as many as 10 nuclear bombs in two to three months — a capacity that the deal will remove. Second, she said, if Iran doesn’t fulfill its commitments under the accord, economic sanctions can snap back in quickly.

But the final point came from the argument that some opponents of the deal are aggressively making now, that negotiators ought to walk away from the negotiation table because a better agreement is possible.

“How this would enhance our ability to negotiate a better deal is a huge question in my mind,” she said. Hirono said she posed the question personally with representatives of the nations co-signing the agreement, “and they all said they would not come back to the table with a different deal.”

In backing the agreement, Hirono joins Hawaii’s other senator, Brian Schatz, who announced his support last Tuesday. Their support is critical to the Obama administration’s political calculus: The GOP-controlled Senate and House are widely expected to pass resolutions against the agreement, which the president would reject, meaning he needs 34 votes in the Senate or 144 in the House to sustain his likely veto.

Sen. Charles Schumer of New York is the only Senate Democrat to publicly oppose the deal thus far, though he is expected to be joined Tuesday by New Jersey Sen. Robert Menendez, who has called a press conference to announce his position.

Neither of Hawaii’s congressional representatives, Tulsi Gabbard or Mark Takai, have announced which way they’ll vote. But the House is considered safer, given that in May, 150 representatives signed a letter supporting the administration’s framework for the deal.

In the Senate, 47 members signed a letter to Iranian religious leaders in March penned by freshman Tom Cotton of Arkansas in an effort to undermine negotiations. Civil Beat was among many news outlets that criticized the move.

Despite that effort, which Hirono described as “beyond the pale,” she believes the Senate will likely sustain the president’s expected veto.

“It’s going to be close,” Hirono said. “I think the votes are there, but I’m not positive.”