Tyler Wigg-Stevenson is chairman of the Global Task Force on Nuclear Weapons for the World Evangelical Alliance. The opinions expressed in this commentary are solely those of the author.

(CNN) It appears that many members of the Senate are fundamentally confused about their role in the international agreement that has been negotiated to block any possible pathway to an Iranian nuclear weapon.

Some senators are acting like the possibilities are wide open: No, not this deal, but perhaps some other deal. But they are missing the point entirely. In fact, what the Senate thinks about the deal by itself is almost irrelevant.

The only question the Senate has to ask and answer is: Now that this deal has been agreed to, is it better than what happens if it is rejected?

Deal opponents, of course, want to hold open the possibility of a rosy, nondeal future. They insist that President Obama's stark dichotomy of deal versus war is a false choice. Not war, they say, but more sanctions! More sanctions will drive Iran to the negotiating table! More sanctions will drive Iran to its knees! Then we will get a deal that we can approve. Wasn't it the President himself, they say, who said that no deal is better than a bad deal?

Maybe this scenario works in the "Iran: Choose Your Own Adventure!" book the GOP caucus appears to be crowdsourcing in its closed-door sessions. In the real world, however, there is precisely zero chance of additional sanctions or new negotiations.

Photos: Look inside Iran: How ordinary people live Will Iran give up its nuclear ambitions? As its leaders try to hammer out a deal with the United States and five other leading world powers, CNN gets rare access to the Islamic state. It's the holy month of Ramadan, which means Muslims fast all day and feast at night. In Tehran, Iranians flock to markets after dark to break their fast. Hide Caption 1 of 12 Photos: Look inside Iran: How ordinary people live Nuts, fresh berries and other fruits are in high demand at this souk, or bazaar, after sunset when the Ramadan fast ends. Hide Caption 2 of 12 Photos: Look inside Iran: How ordinary people live Market stalls heave with fresh fruit in Tehran, aiming to satisfy observant Muslims who have been fasting all day. Hide Caption 3 of 12 Photos: Look inside Iran: How ordinary people live While the shops are full of fruit, other Iranian industries have it harder because of decades of international sanctions. At the Modern Technic factory in Tehran, workers mold auto parts by hand or with only basic machinery. Hide Caption 4 of 12 Photos: Look inside Iran: How ordinary people live Despite its name, the Modern Technic factory is much less automated than most modern auto plants. Most of the machinery is Iranian-made or comes from Russia. Hide Caption 5 of 12 Photos: Look inside Iran: How ordinary people live The Modern Technic auto parts factory still uses equipment made in the Soviet Union because sanctions make it difficult to get new machinery. Hide Caption 6 of 12 Photos: Look inside Iran: How ordinary people live Ayatollah Ruhollah Khomeini led Iran for a decade after the Islamic Revolution of 1979. Today, Iran has a shrine to honor him in Tehran. Hide Caption 7 of 12 Photos: Look inside Iran: How ordinary people live There's space for hundreds of worshippers inside the shrine to Khomeini, with copies of the Quran laid out for each of them. Hide Caption 8 of 12 Photos: Look inside Iran: How ordinary people live The caretakers of the shrine are known as khodam. Hide Caption 9 of 12 Photos: Look inside Iran: How ordinary people live The dome of the shrine over Khomeini's mausoleum. The exterior of the newly constructed dome is still under scaffolding, the final golden tiles being laid. Hide Caption 10 of 12 Photos: Look inside Iran: How ordinary people live Children play in the shrine while their mothers pray or pay their respects to the late ayatollah. Hide Caption 11 of 12 Photos: Look inside Iran: How ordinary people live The heart of the shrine is the tomb of Khomeini, who died in 1989 but is still revered by his followers. Hide Caption 12 of 12

Here's the reality: After 20 months of hard negotiating by the United States, United Kingdom, France, China, Russia and the European Union, the deal with Iran exists. More to the point, outside the Republican caucus and Benjamin Netanyahu and his associates, it has been widely acclaimed as a good deal. This is not some obvious lemon that people are only grudgingly supporting.

So, given this global support, let's imagine for a moment that the Senate rejects this good deal, with a veto-proof majority. What then? Several things happen in relatively short order.

First, the U.S. allies (United Kingdom, France, European Union) and uneasy partners (Russia, China) walk away from the sanctions regime that brought Iran to the negotiating table in the first place. Unlike the United States, which has been politically and economically estranged from Iran for decades, these nations have major economic interests in doing business with Iran.

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Can we possibly imagine that, after negotiating for nearly two years to achieve a diplomatic outcome that they are enthusiastic about, they will continue to uphold watertight sanctions simply because Republicans in the U.S. Senate object to the document?

Even if the United States could persuade the Europeans to hold the line -- and that's"a big if" especially given France's financial interests in Iran -- Russia and China would be all too eager to dissolve sanctions and blame America's domestic politics.

Deal opponents are quick to point out that up-front sanctions relief means Iran could get an influx of cash and then cheat on their nuclear program. But the sanctions regime was dead the moment a deal was signed. If the United States backs out now, Iran still gets the influx of cash, but without having made any promises about their nuclear program and without any arrangements for international monitoring. It's the worst of both worlds.

Second, as for more negotiations -- well, why on Earth would anyone but the United States be interested in doing that? There's no economic incentive, with the sanctions gone. But even worse is the fact that a Senate rejection of the deal would effectively be a declaration that America is a nation without a foreign policy -- that it is in fact incapable of a foreign policy -- because there is no one to talk to whose word at the negotiating table means anything. What the executive giveth, the legislative taketh away. That's no way to do business.

Third, there is no third. Everyone thinks this is a good deal. If America turns it down, we are the only ones who look bad. The sanctions fall apart. Iran gains standing in the international community: they were willing to negotiate, but the United States couldn't take yes for an answer.

After that -- well, who knows? Who knows what Iran will do? Who knows what intelligence will turn up? What we do know, though, is that the possibilities for the peaceful resolution to any future crises will be severely limited, because of bad blood and mistrust over the failed deal.

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This leaves a military response. I do not say "military option" because that language perpetuates the false trope -- pervasive in American political culture -- that, when push comes to shove, the U.S. Air Force is the most efficacious hammer for pounding down any foreign policy nail. In this paradigm, you try diplomacy first and then, when all else fails, use the ever-reliable bombs and guns to achieve your aim.

In fact, the only military way with a chance of permanently foreclosing on an Iranian bomb is a full-scale invasion, takeover, and occupation -- which the United States would have to do virtually alone, having lost any moral authority to command a coalition around this particular problem.

And the Senate GOP, along with Democrats Chuck Schumer and Robert Menendez and their fellow defectors, would be left holding nothing in the smoking remains of America's reputation and power.