A man holds up a sign in New York City on Wednesday at a rally against the nuclear deal with Iran. (Mike Segar/Reuters)

Let’s imagine that the opponents of the nuclear agreement with Iran get their way: The U.S. Congress kills it. What is the most likely consequence? Within one year, Iran would have more than 25,000 centrifuges, its breakout time would shrink to mere weeks and the sanctions against it would crumble. How is this in the United States’ national interest? Or Israel’s? Or Saudi Arabia’s?

This is not an implausible scenario; it is rooted in facts. In 2005, three European powers rejected a nuclear deal with Iran after two years of negotiations. Hassan Rouhani, now president, was then Iran’s chief negotiator. After the talks collapsed, the Islamic republic ramped up centrifuge production, going from fewer than 200 installed to 20,000 today. It also built up more than 16,000 pounds of enriched uranium gas and accelerated work on the heavy-water reactor at Arak, which provides a path to a plutonium bomb.

There is no doubt that Iran has the capacity to make centrifuges, even under crippling sanctions. Between November 2012 and November 2013, when all international sanctions against Iran were in place, it installed 6,000 new centrifuges. Iran’s program has grown through the years with indigenous science and technology, not large-scale reliance on foreigners.

The idea that China, Russia and the European Union would maintain sanctions against Iran if Washington turned down a deal that they painstakingly negotiated and fully embrace is far-fetched. China is desperate to buy Iran’s (discounted) oil. Russia is already negotiating to sell it nuclear-power technology and machinery. And the French foreign minister has scheduled a trip to Tehran next week, presumably to do what that country’s diplomats always do: promote French corporate interests.

It is worth recalling that when the Obama administration was putting together the last round of U.N. sanctions against Iran, many Republicans dismissed the effort. In an August 2009 op-ed in the Wall Street Journal titled “Sanctions Won’t Work Against Iran,” the Bush administration’s ambassador to the United Nations, John Bolton, argued that the other major powers would never go along with such sanctions — and if they did, they wouldn’t change Iran’s behavior anyway. Now Republicans say that these same sanctions are wondrously effective, if only the administration would keep them on indefinitely.

President Obama took questions on the deal with Iran in the East Room of the White House on July 15. Here are key moments from that speech. (AP)

The crucial reason the sanctions have been so effective — more than critics expected — is that they are comprehensive. Leaky sanctions, especially when the leaks are in major countries such as China, Russia and India, are worthless, perhaps even almost counterproductive. They don’t inflict much pain on the regime and actually benefit the hard-liners who control the few gateways in and out of the economy.

There is a profound gap between the United States and the world in the perception of the sanctions against Iran. For many in the United States, the sanctions are a mechanism to punish an evil regime. But for most of the other countries involved, the sanctions were enacted specifically to bring Iran to the negotiating table. These countries would not allow them to be turned into a permanent mechanism to strangle Iran. They all have relations with Iran, traded with it pretty freely until 2012, and intend to resume and expand these ties.

Finally, some who argue against the deal believe that the United States should simply stand firm and Iran will either cave or crumble. Anyone who has dealt with Iranians knows that they are a proud, nationalistic people. The Islamic republic has endured three decades of U.S. sanctions, a nine-year war against Iraq (in which Saddam Hussein used chemical weapons against the Iranians) and other international pressures. If tiny Cuba and North Korea haven’t caved after decades of much greater isolation, it is hard to imagine Iran doing so.

As for the belief that Iran will collapse soon, there is little evidence for this hope. More important, a more democratic Iran would likely still support a nuclear program. In fact, the leader of the 2009 Green Movement, Mir Hossein Mousavi, argued that President Mahmoud Ahmadinejad was making too many concessions to the West regarding Iran’s nuclear rights.

Obama’s critics say he is gambling that Iran will comply with the accord. In fact, the administration is making a calculated bet that Iran will be constrained by international pressure, intrusive inspections, verification mechanisms and the prospect of snapback sanctions. The deal’s opponents have conjured up a fantasy scenario in which the world will sign up for more sanctions, Tehran will meekly return to the table with further concessions, or perhaps the Islamic republic will itself implode — and its successors will then denounce and dismantle the nuclear program. To bet on this scenario is the real gamble, a high stakes one with little evidence to support it.

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To read more on this topic:

Here are key moments from a hearing where Republican senators expressed their displeasure with the agreement on Iran's nuclear program. (Sarah Parnass/The Washington Post)

John F. Kerry and Ernest Moniz: The case for the Iran deal

Michael Gerson: A reckless bet on Iran

Dana Milbank: Republicans’ knee-jerk hatred of the Iran deal

Scott Clement: Obama starts out ahead