The GOP candidates are pledging to undo the Iran nuclear deal. Don't buy it.
Republicans can talk the talk, but history shows they're unlikely to walk the walk
Republicans have spent much of the last six years shaking their fists in impotent protest against the things that Barack Obama has done. That's the way it is when you're out of power: There are only so many tools at your disposal to undo what the president does, even if you control Congress. This dynamic also explains some of the restiveness in the Republican electorate, since their leaders have been telling them of all the ways they'll fight Obama (like repealing the Affordable Care Act), only to be stifled at every turn.
And now it looks like they're going to fail to stop the deal the United States and five other world powers negotiated with Iran to restrain its nuclear program. Since the agreement isn't a treaty, it doesn't require ratification; instead, Congress can try to pass a resolution to stop it, which President Obama would veto. A veto override would require two-thirds of the members of both chambers of Congress, and the deal's opponents aren't going to get that.
While there are still a few Democratic senators who have not made their positions known, the last few days have seen one after another come out in favor of the deal (with the exception of New Jersey's Bob Menendez, to no one's surprise). Republicans need 13 senators to join them in opposition to the deal, and so far they have only two. As of this writing, there are 13 Democrats who have yet to announce their position; unless 11 of them come out in opposition — which seems all but impossible — the deal will have enough supporters to stop a veto override. Furthermore, such an override would probably fail in the House anyway.
So what will happen then? When all the votes are cast and the deal's critics come up short, the Republicans running for president will rush to the microphones to repeat what they've already said: that this is the worst deal in diplomatic history, that Barack Obama is Neville Chamberlain, that Israel is all but already consumed in a fiery nuclear blast, etc.
If there's been any disagreement between the candidates, it's only in how fast they want to tear up the deal. For instance, Scott Walker says he'd do it on "day one" of his presidency, and even suggested he might launch a military strike on Iran to boot. Marco Rubio has said something similar, that he would "quickly reimpose sanctions," which means tossing out the deal. Jeb Bush suggested that he'd at least hire his cabinet and check in with allies before figuring out what to do next, which is what passes for thoughtfulness in GOP circles these days.
What none of them have grappled with is what happens afterward. It's possible that the other signatories to the agreement, including Germany, China, and Russia, will say that whatever President Trump thinks, they'll hold up their end. If Iran agrees, then it might be subject to renewed U.S. sanctions, but the reason the current sanctions regime has been so effective is that the U.N. and so many other nations have participated in cutting Iran out of the world economy; sanctions by the U.S. alone would not have nearly the same impact.
On the other hand, if the agreement falls apart when we pull out — which is what Republicans would obviously prefer — then we return to the status quo, with Iran free to pursue nuclear weapons if it wishes without any inspections at all.
If the past is any indication, I don't expect Republicans to find the time to discuss what would actually happen if they got their wish, since they'll be too busy throwing Munich analogies around. But let's assume that the deal doesn't get shot down in Congress, and it begins to take effect. A year from now, what will the GOP nominee say about the deal? What if it seems to be working — the sanctions have begun to be unwound, inspections are proceeding, and there's no indication yet that Iran is secretly trying to create nuclear weapons. What then? Will that nominee say, "I don't care if it looks like it's working, Bibi Netanyahu once showed me a picture of a cartoon bomb, so I'm still going to walk away from this agreement"?
Maybe. But the truth is that the next president abandoning this agreement has about as much likelihood of happening as Donald Trump's plan to convince Mexico to pay for a 2000-mile wall between our two countries. It's the kind of thing a candidate says when he wants to sound tough, but it's not the kind of thing a president — even if it's one of these guys — actually does. It would get us virtually nothing, and potentially cost us a great deal.
Think about that when you see the candidates shouting at the cameras after Congress fails to stop the agreement, pledging to do their utmost to destroy it.