The Republican chorus gets ever louder: Walk away from an Iran nuclear deal. But of course there is a deal in place, an interim one, much derided by that same chorus when it was concluded in November 2013. At the time, Bob Corker of the Senate Foreign Relations committee lambasted the accord as requiring �no sacrifice on their part whatsoever.�

Now Corker, a Republican of Tennessee and the committee�s chairman, seems to think it�s good enough to leave in place for the moment, holding back criticism of it even as he urges President Obama to avoid a �bad deal� that would hurt �the United States, the region and the world.� The change of tune is not surprising. The interim agreement, respected to the letter by Iran, has proved a milestone.

It has curtailed the country�s nuclear program in a way not seen in many years. Instead of steadily adding centrifuges, the pattern before Obama�s diplomacy, Iran has stopped installation, eliminated or diluted its 20-percent-enriched uranium, and permitted intensified international inspection, among other measures. It has proved Corker�s prediction of �no sacrifice� dead wrong.

This is instructive. It does not mean Iran is to be trusted. It does mean that hard-nosed agreements with Iran can stick and that Tehran must be taken seriously in its declared readiness to reach a fair deal with the United States and its partners. It makes nonsense of Florida Republican presidential candidate Marco Rubio�s statement that the Vienna talks are a �diplomatic charade.�

All the overblown doomsday criticism is easy because the prospective deal is not perfect � diplomacy does not do perfection � and because the Islamic Republic is a hostile power with a record of deception. The tough but worthy endeavor is preventing a nuclear-armed Iran through a rigorous, unambiguous and enforceable agreement that also brings a hopeful, young, highly educated nation closer to the world.

The bottom line is that none of the critics of an Iran deal, from Israeli Prime Minister Benjamin Netanyahu to all the harrumphing Republican presidential hopefuls, has offered a single credible alternative that accomplishes even what has been achieved since 2013.

Absent an accord, Iran will in time resume where it left off 20 months ago. The United States, under Obama or his successor, is not about to go to war with Iran; forget about it. We�ll get the next facile metaphor along the lines of Netanyahu�s warning that an Iranian nuclear threat is coming �to a theater near you,� and another crescendo of rhetoric designed to disguise helpless navel-gazing and, perhaps, a touch of remorse for the opportunity squandered to ring-fence and cut back Iran�s nuclear program under relentless inspection.

There is a transformative opportunity. It will not last long. Iran is vulnerable, economically squeezed, in unusual and delicate equilibrium between its hard-line and reformist forces. What Iran is not, and will never be, is weak enough to be brought to its knees on a core issue of national pride and prestige. Ownership of nuclear know-how (which cannot be bombed out of existence) is as important to Iranians as ownership of its oil was in the early 1950s, before an American-instigated coup. It�s worth recalling that this July marks the 27th anniversary of the shooting down by a United States warship of an Iranian civilian plane with almost 300 people on board. It�s not just for Americans that any accord involves a big psychological hurdle.

There�s a good deal to be had. The opportunity must not be squandered. The deal is not yet in place but enormous obstacles have already been overcome since secret U.S.-Iranian talks began and a productive Washington-Tehran relationship was established for the first time since 1979.

The outstanding issues include unfettered access for International Atomic Energy Agency inspectors to all Iranian sites, including military sites; the sequencing of sanctions lifting; the permitted scope of Iranian nuclear research; and the fate of the arms embargo on Iran. Of these, the first is the most intractable. Obama cannot settle for less than unambiguous Iranian acquiescence to full site access. On the Iranian side, only the supreme leader, Ayatollah Ali Khamenei, can grant that. He has said he won�t. Then again, he has said many things and talks have proceeded. Khamenei knows how much the vast majority of Iranians want this door-opening accord, and how critical it is to a battered economy. His absolute power does not make him politically immune.

Both sides probably have a few weeks to play with. But to imagine the interim deal will hold, absent a final accord, is folly. America�s coalition will fray; Russia and China will start the blame game; Iran will eventually start installing new centrifuges again; the politics of Iran and the United States will shift; Israel will take its brinkmanship an inch or two further; and the hooded, throat-slitting barbarians of Islamic State � enemies of Shiite Iran and the United States � will advance, kill and plunder, relieved of the one conceivable effective coalition to confront them.

This article was written by Roger Cohen for the opinion page of The New York Times on July 9, 2015. Roger Cohen joined The New York Times in 1990. He was a foreign correspondent for more than a decade before becoming acting foreign editor on Sept. 11, 2001, and foreign editor six months later.