Former Vice-President Dick Cheney has joined the debate over the deal with Iran, scheduling a high-profile address and publishing a critical article in the Wall Street Journal. PHOTOGRAPH BY DOUG MILLS / THE NEW YORK TIMES / REDUX

The riskiest gamble of the Obama Presidency, the nuclear agreement with the Islamic Republic of Iran, was basically won today. A rancorous congressional debate is still to come, but thirty-four senators have now vowed to support the deal, effectively blocking efforts on the Hill to eventually kill it. The diplomacy will almost certainly be the centerpiece of Obama’s foreign-policy legacy.

Opponents haven’t given up. The Republican Presidential candidate Donald Trump, the Texas senator Ted Cruz, and the conservative talk-show host Glenn Beck are planning a “Stop the Iran Deal” rally at the Capitol, on September 9th, the day after Congress returns from recess and begins to debate the terms. The rally is co-sponsored by Tea Party Patriots, the Zionist Organization of America, and the Center for Security Policy. It’s likely to be as colorful as it is combative. On “Meet the Press” last month, Trump warned that the deal would be a boon to the Iranians. “They are going to be such a wealthy, such a powerful nation,” he said. “They are going to have nuclear weapons. They are going to take over parts of the world that you wouldn't believe. And I think it's going to lead to nuclear holocaust.”

Former Vice-President Dick Cheney has joined the debate. He is scheduled to give a major address on Iran in Washington next week, at the American Enterprise Institute. “Nearly everything the president has told us about his Iranian agreement is false,” Cheney wrote on Friday, in a Wall Street Journal piece adapted from his book. “He has said it will prevent the Iranians from acquiring nuclear weapons, but it will actually facilitate and legitimize an Iranian nuclear arsenal. He has said this deal will stop nuclear proliferation, but it will actually accelerate it, as nations across the Middle East work to acquire their own weapons in response to America’s unwillingness to stop the Iranian nuclear program.”

The focus in Washington is already shifting, however, to whether the Democrats might be able to win seven more endorsements—for a total of forty-one—to invoke a filibuster that would prevent the deal from even coming to a vote in the Senate. Republicans, with limited Democratic support, have been hoping to pass a Resolution of Disapproval in both the Senate and the House by September 17th. Obama pledged that he would veto it. Support from thirty-four senators now means that the President would be able to sustain his veto—if it comes to that.

“We’ve always said that our goal was for the Administration to be able to implement this deal—whatever that took in Congress,” a senior Administration official told me Wednesday. Taking a swipe at Republicans, the official added, “We appreciate those who have taken this seriously, as opposed to, for example, the forty-seven senators who announced their opposition in a letter to the Ayatollah, months before the agreement was even finalized.”

So far, the prospects of the Democrats blocking a vote altogether are sketchy. Even support for the nuclear agreement—announced on July 24th, after twenty months of tortured diplomacy between Iran and the world’s six major powers—is either tepid or reluctant in both houses of Congress. Senator Barbara Mikulski, a Democrat from Maryland who retires next year, became the decisive thirty-fourth voice in the Senate on Wednesday. Her views reflected those of many fellow-Democrats.

“No deal is perfect, especially one negotiated with the Iranian regime,” Mikulski said. “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.” She, like many others, had caveats. “However, Congress must also reaffirm our commitment to the safety and security of Israel,” she said.

Democrats have forty-four seats in the Senate. Ten members have not yet declared how they will vote. The lobbying has been ferocious, the media blitz of anti-deal television ads sometimes apocalyptic. Two Democratic senators—Charles Schumer, of New York, and Robert Menendez, of New Jersey—have announced opposition to the deal. The New Jersey Democrat Cory Booker is among those facing particular pressure from constituents and donors on both sides. Maryland’s Ben Cardin is also reportedly torn.

Secretary of State John Kerry, in a push to mobilize more votes, sent separate letters to the Senate and the House today, pledging to work with Congress on new legislation that would guarantee the security of allies, notably Israel and the Gulf countries, as well as check Iran’s behavior in the region. Israel’s security, Kerry wrote, is “sacrosanct.”

Kerry also made an emotional final pitch for the deal, before congressional debate begins next week, in a speech in Philadelphia. “The outcome will matter as much as any foreign-policy decision in recent history,” he said. “To vote down this agreement is to solve nothing. . . . To oppose this agreement is—whether intended or not—to recommend . . . a policy of national paralysis. It is to take us back directly to the very dangerous spot that we were in two years ago—only to go back there devoid of any realistic plan or option.”

The White House has long claimed that the deal with Iran was about only one issue and does not represent an attempt at rapprochement between Washington and Tehran. But the Administration’s arguments increasingly hint at future possibilities. “The Iran agreement is not a panacea for the sectarian and extremist violence that has been ripping that region apart,” Kerry said in Philadelphia. “But history may judge it a turning point, a moment when the builders of stability seized the initiative from the destroyers of hope, and when we were able to show, as have generations before us, that when we demand the best from ourselves and insist that others adhere to a similar high standard—when we do that, we have immense power to shape a safer and a more humane world.”

Iranian officials have begun using similar language to sell the deal. Ali Larijani, the Speaker of Parliament, is in New York for meetings at the United Nations. In an interview with CNN’s Christiane Amanpour on Tuesday, he called the agreement “a beginning for a better understanding for other issues as well.” A more “realistic approach and attitude” by the United States toward Iran would be reciprocated, he said. Larijani is a powerful figure and a conservative. The son of a grand ayatollah, he is a former Presidential candidate. One of his brothers heads the Iranian judiciary, while another heads a human-rights council. Larijani also met Tuesday with a few former congressmen and discussed possible exchanges, according to two people who were present at the meeting.

The White House hopes that the congressional debate will be resolved before the opening of the U.N. General Assembly at the end of September, when President Obama and Iran’s President Hassan Rouhani are both scheduled to speak, on the same day.