The best chance to ensure that Iran’s nuclear program remains peaceful is imperiled because of mistaken notions about what real alternatives the West has.

There certainly is an agreement to be had that is consistent with the preliminary accord, known as the Joint Plan of Action, reached more than a year ago. This plan placed major restrictions on Iran’s nuclear program and subjected it to an unprecedented degree of monitoring. But the whole negotiating edifice would crash if the U.S. Congress either rejects a final agreement, with a resolution of disapproval, or decides to impose new sanctions on Iran, which would violate, and thus kill, the preliminary accords.

If this happens, Iran’s nuclear program would be unrestricted and unmonitored — apart from what applies to any state that is a party to the Nuclear Nonproliferation Treaty and to standard agreements with the International Atomic Energy Agency. Indefinitely extending the preliminary accord could serve nonproliferation objectives. But this is not politically feasible in either Tehran or Washington.

The Iranians could not be expected, for example, to prolong endlessly an arrangement that offers only minimal sanctions relief. Congressional hawks, meanwhile, have made it clear they would impose additional sanctions if a final agreement is not reached by early summer.

Members of Congress who seem primed to oppose whatever agreement emerges from the negotiations usually base their opposition on the idea that rejecting the agreement would clear the way for a “better deal.” That belief is a fantasy.

If the agreement reached in these negotiations is killed, there probably won’t be another chance for many years. There is no mysterious process that could cause a better deal to materialize.

As for imposing additional sanctions, there is nothing in the Iranians’ record to suggest that at some level of economic pain they would cry uncle and capitulate to hard-line demands. If this were possible, it would have happened by now after many years of debilitating sanctions. Just as would be the case with Washington, there is no political or psychological room in Iran for capitulation under that kind of pressure.

The principal source of the Iranians’ doubt and hesitation in these nuclear talks has not been what would happen if there is no agreement. It is instead what they expect to happen if there is an agreement.

Congressional opposition has given Tehran plenty of reasons to doubt Washington’s willingness to strike a workable deal, and even to doubt the Obama administration’s ability to fulfill the U.S. end of the bargain. Any new deal-killing action by Congress would confirm the Iranians’ doubts and lead more of them to conclude that there is no use trying to do business with Washington.

Additional political responses in Tehran would make prospects for striking a new and different deal even more remote. President Hassan Rouhani, who has staked his political career on the success of these negotiations, would become a discredited lame duck. The supreme leader, who has supported Rouhani’s efforts so far but has left himself an out by repeatedly expressing doubt about U.S. intentions, would say, “I told you so.” The political power of Iranian hard-liners, who are less, not more, willing to make fresh concessions on the nuclear issue, would increase.

As for one other form of pressure that those holding out for a better-deal fantasy often suggest — the threat of military attack on Iran: It would be hard to imagine anything — other than an actual attack — better designed to rekindle whatever lingering interest there is in Iran to develop nuclear weapons as a deterrent.

Assessing the product of any negotiation, on this or any other topic, requires comparing it to its alternative. The alternative here is no agreement. A good deal would be one that is better than no deal; a bad deal would be one that is worse.

Members of Congress who oppose an agreement would, in effect, be casting a vote in favor of allowing Iran to run as many centrifuges as it wants; to accumulate unlimited stockpiles of enriched uranium, and to resume enrichment at the higher levels it has previously abandoned. It also would be a vote to remove additional international inspectors placed in Iran under the preliminary accords.

Anyone who casts a vote with these effects will have a lot of explaining to do to constituents.