A disaster has been averted. The nuclear deal concluded by six major powers with Iran is now unstoppable in Congress. The only question is whether President Obama will have to veto a Republican resolution of disapproval, or whether Democrats will have enough votes to spare him that obligation by filibustering the resolution and ensuring it never leaves the Senate.

The second outcome would be preferable. An override-proof presidential veto of a congressional resolution of disapproval is not the best path to a historic international accord. Major arms control treaties used to be bipartisan affairs. Those days are gone. Still, Republican maneuvering, backed by Prime Minister Benjamin Netanyahu of Israel, has not prevailed. That’s a victory for reason in a season of rage.

Why has a disaster been averted? Because if the deal had unraveled in Congress, so would America’s standing as a global power. Russia, China and the European Union would have concluded that the United States is unserious. To negotiate over years a tough compromise obliging Iran, among other measures, to slash its stockpile of enriched uranium by 98 percent and its operating centrifuges by two-thirds, and then walk away in a righteous and deluded funk . well, that’s not how America won the respect of the world. It did so by being consequential in hot wars and cold.

Perhaps American power matters less these days. I don’t think so. If America’s word is shot, so are the underpinnings of global security. What would have been left of that word after an Iranian volte-face . to follow the Syrian volte-face and ongoing debacle . would have been meager indeed. America the Undependable would have become a watchword.

Then there’s Iranian power. Absent the nuclear deal, it would have grown. Tehran would have secured its objective of relief from sanctions . because the current coalition would have crumbled . without having to scale back its nuclear program or submit it to international inspection of unprecedented intensity.

Iran would have resumed installation of new centrifuges and increased the quantity and quality of its enriched uranium, just as it did for many years before Obama’s diplomacy reversed things. As Secretary of State John Kerry remarked this week, rejection would have left the United States and its allies in “the very dangerous spot that we were in two years ago” . only “devoid of any realistic plan or option.” Of course, one option would have resurfaced . the military one, a certain catastrophe with a maximum setback of Iran’s nuclear program of perhaps three years, against 15 years in the deal without a third unwinnable American war certain to convince every wavering Iranian of the need for a nuclear weapon.

There was no “better deal” . the fantasy of all those who hate Iran and hate Obama (which of them more is often unclear). The nuclear deal has become “such a luscious piece of Republican propaganda,” William Luers, the director of The Iran Project, whose goal is to improve American-Iranian relations, told me. And a long election season has already begun.

I said a disaster had been averted. A disaster has also been revealed. It is that not one . not one . of the 301 Republican members of Congress (the largest Republican majority since 1929-31) supports the deal, despite the overwhelming evidence that the accord, while far from perfect, is the best achievable . as almost all Democrats concluded after often agonized review.

Republicans, prodded by Mr. Netanyahu and the American Israel Public Affairs Committee (in overdrive on the Iran question), are not about to let go of their Iran toy. It’s a way to attack the president; it’s also a way to raise money. The deal will become the “Obamacare of foreign policy,” Nicholas Burns, a Harvard professor and former under secretary of state, told me. Yes, it will. That is, something sensible (at least in the eyes of most people across the world) to which Republicans will never acquiesce and which they will try to use in every conceivable way to undermine a president they loathe.

Prepare for congressional attempts to step up enforcement of non-nuclear related Iran sanctions, increase military aid to Israel over the next decade, constrain Iran’s options in spending the windfall from sanctions relief, bolster Persian Gulf allies, and so on. There’s a good case for doing much of this . as Obama himself has argued. But that case gets undermined when the underlying Republican objective is to sabotage the nuclear deal. Implacable hostility to Iran will be met in kind.

Aipac recently put out a memo called “Promises Cannot Fix a Bad Deal.” In essence it said nothing Obama promises Israel will help because the deal’s “fatal flaw” is it “legitimizes Iran as a nuclear-threshold state in 15 years.”

Here’s some breaking news for Aipac: Iran is a nuclear-threshold state today. The great merit of the deal is to slash and ring-fence acquired Iranian nuclear capacity until 2030. To say that is not in Israel’s security interest is utterly unpersuasive.