Republican presidential candidates are climbing over one another to denounce the Iran nuclear deal. Jeb Bush called it a "terrible deal." Marco Rubio labeled it "a dangerous and destabilizing failure." Ted Cruz warned it "will result in the United States government becoming one of the leading funders of international terrorism." Rick Perry pledged to kill it as "one of my first official acts" as president; Scott Walker said he would "very possibly" order airstrikes on Iran on his first day in office.

This all makes political sense. But don't take it too seriously as actual policy. The truth is that Republicans are in no real position to kill the Iran deal — probably not in Congress, and probably not even if they take the White House in 2016. Nor, really, should they want to: At this point, even if you are an Iran hawk who hates the deal, unilaterally torpedoing it would hurt your goals rather than advance them.

Republicans have an Iran problem: They are politically wedding themselves to something that is, in practice, going to be very difficult or impossible for them to do. Unless something substantial changes on the ground — maybe Iran is caught in a major violation and refuses to fix it — American opponents of the Iran deal are probably not going to be able to kill it.

Congressional Republicans are in a very weak position to kill the deal

The 2015 Iran Nuclear Agreement Review Act gives Congress 60 days to review the deal before Obama can begin lifting sanctions. In theory, they could vote to block US sanctions relief, which would violate the terms of the deal — effectively killing it.

But that almost certainly won't happen. Congress is "effectively powerless to stop the Iran deal," writes Jack Goldsmith, the Henry L. Shattuck Professor at Harvard Law School.

The simple explanation is numbers. Republicans would have to overcome a presidential veto. For that, they need every single Republican, plus 13 Senate Democrats and 44 House Democrats to vote in their favor — which, as Andrew Prokop points out, is a full fifth of the Democrats in each chamber.

That's a huge hurdle. Even if you remove the actual foreign policy considerations, it would be politically bizarre for Democrats to so hurt their own party. This is President Obama's signature foreign policy achievement; over the course of 2015, supporting an Iran deal has increasingly become identified with supporting the president's foreign policy. Hillary Clinton, Democratic heir apparent, is on board with the deal — as is House Minority Leader Nancy Pelosi, who wields tremendous influence over House Democrats. Democratic voters support it by a whopping 69-25 margin, according to a Washington Post poll. The margin is pretty large among the general public, as well: 56 to 37.

On foreign policy terms, killing the deal now is what you might call a tough sell. The most likely outcome, if the US Congress kills the deal, is that the international sanctions coalition against Iran would collapse. Keep in mind that the US signed this agreement not just with Iran but also with the UK, France, Russia, China, and Germany — all of whom would blame the US for breaking its word. Iran would suddenly be free from its obligations under the deal to dismantle its nuclear program. We would go from severe restrictions on Iran's nuclear program to effectively no restrictions, and we would go from a crippling sanctions regime to a very weak one. Even if you hate the Iran deal, that scenario is a lot worse.

So in order to even have a chance of killing the Iran deal, Republicans will have to convince a large number of Democrats to abandon their president on his biggest foreign policy initiative, in defiance of the party's voters and top leaders — to support a bill that would be likely to end up accomplishing the opposite of what it's supposed to. That's a tough sell.

Republicans' Iran problem only gets harder after Obama leaves office

As the years drag on, it will become harder for Republicans to unilaterally kill the deal — even if the Republicans take back the White House. By the time President Scott Walker or President Marco Rubio would be inaugurated, most of the international sanctions will already be gone. The whole point of killing the deal right now is to keep sanctions in place. Meanwhile, the international community will have a bunch of inspections and monitoring in place that the US would jeopardize by killing the deal.

"The next president can start from scratch," Bob Corker, chairman of the Senate Foreign Relations Committee, told Reuters. "What would have happened, though, is the international sanctions process would have been totally dismantled." Merely withdrawing from the deal, in other words, isn't enough: A new Republican president would need to figure out some way to convince the world to put sanctions back in place.

But it's really, really hard to imagine how the US could convince Russia and China and Europe to go back on a nuclear deal they so clearly see as a good thing. The entire point of the sanctions regime was to get Iran to agree to limits on its nuclear program. From the world's perspective, Iran has just agreed to put limits on its nuclear program.

No major power would go along with a new American president who wants to kill the deal and roll the clock back to 2013. Destroying the deal would both 1) give Iran sanctions relief and 2) remove the strict limits the deal puts on Iran's nuclear program. That's the best of both worlds for Tehran — and the worst for Washington.

Some of the more forthright hawks recognize this. "If the next president is to have any hope of putting Iran back into the sanctions box, he or she will have to do some heroic diplomatic work to convince our allies to go along or else risk open economic warfare with our closest allies," Max Boot, a leading Iran hawk at the Council on Foreign Relations, writes in Commentary.

Boot counsels a future Republican president "to keep the accord in place" while "building the case, both at home and abroad, for re-imposing sanctions and even using force if necessary to stop the Iranian nuclear program."

It's hard to imagine the world powers deciding that it should join a President Walker or Rubio in reneging on the deal and punishing Iran for its compliance (if Iran cheats, the deal has mechanisms for reimposing sanctions). The fact that this is where the anti-Iran-deal policy conversation is — super long-shot schemes premised on convincing all the world powers to betray a deal they just signed — tells you a lot about the likelihood of actually killing it.

The deal is likely sealed

There are other imaginable avenues to killing the deal, but they're not much more attractive. A Republican president could simply launch a war out of thin air, as Walker suggested, but that'd be a disaster; the rest of the world would correctly see it as America betraying its commitments to the nuclear deal. The backlash would be even worse than it was to the 2003 Iraq invasion. And if a Democrat wins the White House, congressional Republicans will be in a worse place than they are now.

It is certainly possible that the deal could fall apart in some other way: Maybe Iran will stonewall inspectors on verifying that it's complied with the terms, for example. But it is just very difficult to imagine any plausibly functional way in which Republicans can now unilaterally kill the deal.

The Iran deal is almost certainly here to stay. Republican presidential candidates will likely say all sorts of things about rolling it back. The party's hawks are a critical constituency in the upcoming primary battle, and saying that you hate the deal and want to kill it helps convince those folks that you share their priorities.

But don't confuse those pledges with, y'know, actual policy commitments. A Republican president will have little choice, like it or not, but to see the deal through.