While many members of Congress have declared their positions on the Iran nuclear deal, a significant number remain undecided.

I and my colleagues in the administration have spoken to many of those still digging into its details, poring over the annexes and asking specific questions�questions like how long traces of uranium would be detectable after a hidden site was �cleaned up� (answer: many lifetimes over) or what the �snap back� of sanctions at the United Nations would mean for businesses that had already signed new contracts in Iran (answer: they would have to phase out their contracts or face penalties for violating sanctions). They are asking these questions in an effort to determine what is best for our national security.

Yet while our discussions delve deep into the details of the deal itself, far less time is focused on how this deal would affect America�s foreign policy heft on issues far beyond Iran�s nuclear program.

Serving as the United States ambassador to the United Nations, where we hold one of five permanent seats of the UN Security Council and are one of 193 member states, I spend virtually every day interacting with foreign diplomats�diplomats who are tracking the debate in Congress like they tracked the World Cup soccer pairings last year. And from this vantage point, I believe that rejecting this deal would significantly weaken our ability to achieve our broader foreign policy goals�most of which in 2015 require us to mobilize broad international coalitions.

First, if the United States rejects this deal, we would instantly isolate ourselves from the countries that spent nearly two years working with American negotiators to hammer out its toughest provisions. Those partners believe that this is a sound deal�with a rigorous set of inspection measures that would allow us to know if Iran is not playing by the rules. And those countries have been very clear that they are not prepared to walk away from this deal to try to secure different terms. So if we walk away, there is no diplomatic door number two. No do over.

No rewrite of the deal on the table. We would go from a situation in which Iran is isolated to one in which the United States is isolated.

That would not be ideal under any circumstances, but it would be particularly damaging in a context in which Iran continues to pose a profound threat to international peace and security, against which global unity and pressure will be critical.

Second, well beyond the consequences vis-a-vis Iran itself, rejecting this deal would likely undermine our ability to use sanctions in other circumstances. At the UN I routinely encounter countries that do not want to impose sanctions or even to enforce those already on the books. The hard-line sanctions skeptics have their own self-interested reasons for opposing sanctions, but they ground their opposition in claims that America uses sanctions to inflict punishment for punishment�s sake.

In response, I tell foreign diplomats that sanctions are not an end in themselves, but a means of marrying coercive measures with diplomacy to try to change behavior�whether that behavior threatens international security or inflicts widespread human suffering. And I tell those diplomats that when diplomatic paths seem to emerge, America will pursue them, and we will ease the pressure if the grounds for imposing sanctions are addressed.

In the case of Iran, the United States persuaded other countries to apply pressure for a purpose�in order to secure significant, long-term constraints that would cut off all of Iran�s pathways to a nuclear weapon. If we move the goalpost now�arguing, for example, that there should not be sanctions relief until Iran stops supporting terrorist proxies or until it permanently gives up nuclear enrichment for peaceful purposes�we would give detractors a powerful tool to try to obstruct our future efforts on issues unrelated to Iran.

Our efforts to reach this deal have affirmed the view of the United States as a tough but principled leader; rejecting it would be read in many quarters as a superpower intent on inflicting pain for its own sake.

So the next time we try to rally countries to join us in imposing asset freezes or travel bans on military leaders whose soldiers commit atrocities�as we succeeded in doing in July for six commanders in South Sudan�we may find it harder to marshal international support. And the next time we try to persuade the UN Security Council to ratchet up sanctions on North Korea following a nuclear weapons test or other provocative actions, we may find other countries less willing to impose stronger measures. Meanwhile, as we try to convince warlords and rogue states targeted by other sanctions regimes to change their ways, they may assume�not without reason�that we will keep punitive measures in place regardless of how they act.

Finally, walking away from this deal may well make it harder for us to rally multilateral coalitions necessary to confront other grave threats�whether those threats come from a regime armed with a nuclear weapon, a deadly virus, or a group of foreign terrorist fighters. These threats do not respect borders and pose a risk to all nations, yet too many countries sit back and expect the United States to shoulder a disproportionate share of the collective security burden. We must get other countries to do their part, but getting them to do so depends a lot on how the United States is perceived in the world.

The Iran nuclear deal has been championed by the president of the United States, every one of America�s European friends and countless other countries around the world.

If Congress rejects the deal, we will project globally an America that is internally divided, unreliable and dismissive of the views of those with whom we built Iran�s sanctions architecture in the first place. Although it is hard to measure the precise impact of these perceptions, I and other American diplomats around the world draw every day on our nation�s soft power, which greatly enhances our ability to mobilize other countries to our side.

While that soft power is built in many ways, two of its most important sources are the belief among other countries� leaders and publics that we share similar values, and that America delivers on its commitments. Of course, there is no substitute for the essential deterrent and coercive effects rooted in the hard power of America�s unmatched military arsenal.

But we should not underestimate the political capital we will lose�political capital that we draw upon for influence�if we walk away from this deal.

Senators and representatives are right to deliberate carefully over whether America�s national security interests are better served by accepting or rejecting this deal. Putting the deal under the microscope is a crucial part of that process. But so is taking a step back and weighing the cost that rejecting the deal would have on our ability to lead the world in confronting Iran and other 21st century threats. Viewed from that perspective, the price of our lonely walk away looks very high indeed.
This article was written by�Samantha Power for Politico Magazine on Aug. 26, 2015. Samantha Power is U.S. ambassador to the United Nations.