The next Republican nominee for president, in all likelihood, will be Donald Trump, Sen. Ted Cruz, or Sen. Marco Rubio. Two-thirds of Republican voters support one of these three candidates. In nearly every Iowa poll, Trump, Cruz, and Rubio lead the pack. In political betting markets, they’re drawing about 90 percent of the money.

Cruz says the nomination is a two-man race between him and Trump. You might not love me, says the Texas senator, but I’m the only guy left who can beat Trump and save the GOP from disaster. Rubio offers a similar pitch: He’s the electable Republican, the only alternative to a lousy choice between Trump and Cruz.

The argument for Cruz or Rubio, in short, is really an argument against Trump: He’s reckless, he’s dangerous, he says crazy things. But there’s a problem with this rationale. As both senators have noted, the most important thing a president can change by himself is foreign policy. In this case, he can revoke American participation in the biggest nuclear nonproliferation pact in almost 20 years: the 2015 agreement between six countries and Iran, which was formally implemented this weekend. And on that issue—the one that arguably matters most—Trump isn’t crazy. Of the three leading Republicans, he’s the only one who hasn’t promised to revoke the Iran deal on his first day in office.

Last March, even before the deal’s terms were settled, Rubio pledged to revoke any agreement in which Iran retained a “capability to enrich uranium.” In August, after the agreement was finished and signed, Cruz vowed to “rip up and rescind this catastrophic Iranian nuclear deal.”

Trump bashed the agreement, too. But he didn’t promise to revoke it. In August, a week after Cruz pledged to rip up the agreement, Trump explained why he wouldn’t do the same:

I’ve heard a lot of people say, “We’re going to rip up the deal.” It’s very tough to do [that] … Because I’m a deal person. And when I make deals … I will police that deal. You know, I’ve taken over some bad contracts. I buy contracts where people screwed up, and they have bad contracts. But I’m really good at looking at a contract and finding things within a contract that, even if they’re bad, I would police that contract so tough that they don’t have a chance. … And the problem is, by the time I got in there, they will have already received the $150 billion.

In September, Cruz and Trump appeared together at a rally against the Iran deal. Cruz told the crowd, “Any commander in chief worthy of defending this nation should be prepared to stand up on Jan. 20, 2017, and rip to shreds this catastrophic deal.” Trump made no such commitment. Two months later, in his book, Crippled America, Trump wrote: “Once the sanctions are removed there is no going back, no ‘snapback.’ Putting sanctions back in place unilaterally won’t do any good.”

Trump was right. Revoking U.S. compliance made no sense. The agreement’s smartest critic, Republican Sen. Bob Corker of Tennessee, explained in July that by the time a new U.S. president took office, “the international sanctions process would have been totally dismantled.” The president could restore American sanctions but not the joint European-Russian-Chinese–United Nations sanctions that had forced Iran to the bargaining table. Iran would have its money, and the Europeans would be in no mood to follow a unilateral American withdrawal from a complex agreement everyone had painstakingly negotiated.

Worse yet, Iran would be free to resume its pursuit of nuclear weapons. The Iranian government, which so far has complied with the agreement, could point to the American withdrawal and announce that the deal was off. Russia and China would be able to say that they had tried to help the West reach a responsible agreement but that the Americans had broken it, and therefore further sanctions were pointless. Even in Europe, the United States would get the blame.

And the damage wouldn’t end there. As Cruz has pointed out in the case of Libya, when regimes don’t get rewarded for relinquishing their nuclear weapons programs, other countries aren’t likely to offer the same concessions. Last month, Cruz complained that the Obama administration had turned against Libyan dictator Muammar Qaddafi after he “voluntarily handed over his nuclear program and shut it down.” In an interview with MSNBC’s Joe Scarborough, Cruz repeated this point and commiserated over the counterproductive message Obama had thereby sent to other regimes.

The far better approach is the one Trump described: Police the deal. Use the elaborate compliance requirements and punitive measures with which the Iran agreement is loaded. If Iran fails to meet any of these terms, cite the text and bring the pain. Keep the Europeans on our side, and hold the Russians and Chinese to their word.

That’s the position some other Republican candidates have adopted. In July, a week before the terms of the deal were announced, New Jersey Gov. Chris Christie refused to take Rubio’s hard line. “I’m not one of those guys who’s going to say to you, ‘On Day One, I will abrogate the agreement,’ ” Christie told an audience in New Hampshire. Instead, Christie said he would “decide what to do with the agreement, depending on where we are at that moment. Because, by the way, it’s not just us involved anymore. We have a number of our allies around the world who are at that table as well, and sanctions are most effective when not only we do it, but the other allies do it.”

Three weeks after Christie spoke, Ohio Gov. John Kasich expressed similar concerns. “When people say they’re going to tear up the deal, what are they going to do then?” Kasich asked on Meet the Press. “Some people are saying, ‘Well, we’re going to go to war.’ … That’s inexperience. That’s just playing to a crowd.” This weekend on Face the Nation, Kasich added: “If I were president today, I would be meeting with every one of our allies around the world and saying, ‘We’re going to monitor this deal. And if they violate one crossed t or one dotted i, we need to slap the sanctions back on.”

That’s how a sober hawk would manage the Iran agreement. But Christie and Kasich have no shot in Iowa. And unless one of them cracks the top three in New Hampshire, they’ll soon be out of the race, leaving us with Trump, Cruz, and Rubio. This weekend, Rubio reaffirmed, “I will repeal the deal with Iran.” And Cruz boasted again that on his “first day in office,” he would “rip to shreds this catastrophic Iran nuclear deal.”

The best scenario is that Cruz and Rubio are lying: They’re throwing red meat to crowds, fully intending to betray their promises. The worst scenario is that they’re serious. Either way, their recklessness on a matter of such gravity is damning. The GOP may soon face a choice between a businessman who understands the importance of honoring deals and two freshman senators who don’t. To save the party from dangerous fools, Republicans might have to nominate Trump.

Katherine Kwok and Anna Weber provided research assistance for this article.

Read more of Slate’s coverage of the GOP primary.