Let's imagine that the opponents of the nuclear agreement with Iran get their way. The United States 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 America's 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 over 16,000 pounds of enriched uranium gas, and ramped up 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, it 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.
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 like 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 America 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 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 the Islamic Republic will collapse soon, there is little evidence for this hope. More importantly, we know that 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 that 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.
Fareed Zakaria hosts CNN's "Fareed Zakaria GPS," and makes regular appearances on shows such as ABC’s "This Week" and NBC's "Meet the Press." He has been editor at large at Time magazine since 2010, and spent 10 years overseeing Newsweek's foreign editions. He is a Washington Post (and internationally syndicated) columnist. He is author of "The Post-American World." For more of Fareed Zakaria's reports, Go Here Now.