There is one foreign policy issue on which the GOP's takeover of the Senate could have huge ramifications, and beyond just the US: Republicans are likely to try to torpedo President Obama's ongoing efforts to reach a nuclear deal with Iran. And they just might pull it off.

November 24 is the latest deadline for a final agreement between the United States and Iran over the latter's nuclear program. That'll likely be extended, but it's a reminder that the negotiations could soon come to a head. Throughout his presidency, Obama has prioritized these negotiations; he likely doesn't want to leave office without having made a deal.

But if Congress doesn't like the deal, or just wants to see Obama lose, it has the power to torpedo it by imposing new sanctions on Iran. Previously, Senate Majority Leader Harry Reid used procedural powers to stop this from happening and save the nuclear talks. But Senate Majority Leader Mitch McConnell may not be so kind, and he may have the votes to destroy an Iran deal. If he tries, we could see one of the most important legislative fights of Obama's presidency.

Why Congress can bully Obama on Iran sanctions

At their most basic level, the international negotiations over Iran's nuclear program (they include several other nations, but the US is the biggest player) are a tit-for-tat deal. If Iran agrees to place a series of verifiable limits on its nuclear development, then the United States and the world will relax their painful economic and diplomatic sanctions on Tehran.

"The regime of economic sanctions against Iran is arguably the most complex the United States and the international community have ever imposed on a rogue state," the Congressional Research Service's Dianne Rennack writes. To underscore the point, Rennack's four-page report is accompanied by a list of every US sanction on Iran that goes on for 23 full pages.

The US's sanctions are a joint Congressional-executive production. Congress puts strict limits on Iran's ability to export oil and do business with American companies, but it gives the president the power to waive sanctions if he thinks it's in the American national interest. "In the collection of laws that are the statutory basis for the U.S. economic sanctions regime on Iran," Rennack writes, "the President retains, in varying degrees, the authority to tighten and relax restrictions."

The key point here is that Congress gave Obama that power — which means they can take it back. "You could see a bill in place that makes it harder for the administration to suspend sanctions," Ken Sofer, the Associate Director for National Security and International Policy at the Center for American Progress (where I worked for a little under two years, though not with Sofer directly), says. "You could also see a bill that says the president can't agree to a deal unless it includes the following things or [a bill] forcing a congressional vote on any deal."

Imposing new sanctions on Iran wouldn't just stifle Obama's ability to remove existing sanctions, it would undermine Obama's authority to negotiate with Iran at all, sending the message to Tehran that Obama is not worth dealing with because he can't control his own foreign policy.

So if Obama wants to make a deal with Iran, he needs Congress to play ball. But it's not clear that Mitch McConnell's Senate wants to.

Congress could easily use its authority to kill an Iran deal

To understand why the new Senate is such a big deal for congressional action on sanctions, we have to jump back a year.

In November 2013, the Obama administration struck an interim deal with Iran called the Joint Plan of Action (JPOA). As part of the JPOA, the US agreed to limited, temporary sanctions relief in exchange for Iran limiting nuclear program components like uranium production.

Congressional Republicans, by and large, hate the JPOA deal. Arguing that the deal didn't place sufficiently serious limits on Iran's nuclear growth, the House passed new sanctions on Iran in December. (There is also a line of argument, though often less explicit, that the Iranian government cannot be trusted with any deal at all, and that US policy should focus on coercing Iran into submission or unseating the Iranian government entirely.) Senate Republicans, joined by more hawkish Democrats, had the votes to pass a similar bill. But in February, Senate Majority leader Harry Reid killed new Iran sanctions, using the Majority Leader's power to block consideration of the sanctions legislation to prevent a vote.

McConnell blasted Reid's move. "There is no excuse for muzzling the Congress on an issue of this importance to our own national security," he said. So now that McConnell holds the majority leader's gavel, it will remove that procedural roadblock that stood between Obama and new Iran sanctions.

To be clear, it's far from guaranteed that Obama will be able to reach a deal with Iran at all; negotiations could fall apart long before they reach the point of congressional involvement. But if he does reach a deal, and Congress doesn't like the terms, then they'll be able to kill it by passing new sanctions legislation, or preventing Obama from temporarily waiving the ones on the books.

And make no mistake — imposing new sanctions or limiting Obama's authority to waive the current ones would kill any deal. If Iran can't expect Obama to follow through on his promises to relax sanctions, it has zero incentive to limit its nuclear program. "If Congress adopts sanctions," Iranian Foreign Minister Javad Zarif told Time last December, "the entire deal is dead."

Moreover, it could fracture the international movement to sanction Iran. The United States is far from Iran's biggest trading partner, so it depends on international cooperation in order to ensure the sanctions bite. If it looks like the US won't abide by the terms of a deal, the broad-based international sanctions regime could collapse. Europe, particularly, might decide that going along with the sanctions is no longer worthwhile.

"Our ability to coerce Iran is largely based on whether or not the international community thinks that we are the ones that are being constructive and [Iranians] are the ones that being obstructive," Sofer says. "If they don't believe that, then the international sanctions regime falls apart."

This could be one of the biggest fights of Obama's last term

It's true that Obama could veto any Congressional efforts to blow up an Iran deal with sanctions. But a two-thirds vote could override any veto — and, according to Sofer, an override is entirely within the realm of possibility.

"There are plenty of Democrats that will probably side with Republicans if they try to push a harder line on Iran," Sofer says. For a variety of reasons, including deep skepticism of Iran's intentions and strong Democratic support for Israel, whose government opposes the negotiations, Congressional Democrats are not as open to making a deal with Iran as Obama is. Many will likely defect to the GOP side out of principle.

The real fight, Sofer says, will be among the Democrats — those who are willing to take the administration's side in theory, but don't necessarily think a deal with Iran is legislative priority number one, and maybe don't want to open themselves up to the political risk. These Democrats "can make it harder: you can filibuster, if you're Obama you can veto — you can make it impossible for a full bill to be passed out of Congress on Iran," Sofer says. But it'd be a really tough battle, one that would consume a lot of energy and lobbying effort that Democrats might prefer to spend pushing on other issues.

"I'm not really sure they're going to be willing to take on a fight about an Iran sanctions bill," Sofer concludes. "I'm not really sure that the Democrats who support [a deal] are really fully behind it enough that they'll be willing to give up leverage on, you know, unemployment insurance or immigration status — these bigger issues for most Democrats."

So if the new Republican Senate prioritizes destroying an Iran deal, Obama will have to fight very hard to keep it — without necessarily being able to count on his own party for support. And the stakes are enormous: if Iran's nuclear program isn't stopped peacefully, then the most likely outcomes are either Iran going nuclear, or war with Iran.

The administration believes a deal with Iran is their only way to avoid this horrible choice. That's why it's been one of the administration's top priorities since day one. It's also why this could become one of the biggest legislative fights of Obama's last two years.