Republicans in Congress will get a chance to kill President Obama's new nuclear deal with Iran. But they'd have to win over a sizable chunk of Democrats to do it. The deal doesn't need to be affirmatively approved by Congress. But a law passed earlier this year allows Congress to vote to disapprove it and therefore block the relief of sanctions on Iran that's crucial to it. However, this resolution of disapproval would need to be passed by both the House and Senate over President Obama's veto. Assuming every Republican in the House and Senate votes to disapprove the deal, 13 Senate Democrats and 44 House Democrats would have to join them to kill Obama's veto. (If a vacant House seat is filled by a Republican by the time of the vote, only 43 House Democrats would be needed.) That would be over a fifth of the Democratic delegation in each chamber. Given that Obama made the deal and Hillary Clinton is praising it, defections like these seem unlikely unless key party leaders, like Sen. Chuck Schumer and House Minority Whip Steny Hoyer, come out against it.

How the congressional review process works

At the start of this year, Congress had no clear way to review the deal with Iran that Obama's team was hoping to strike. Both Democrats and Republicans were unhappy about this, and so after some debate, the bipartisan Iran Nuclear Agreement Review Act of 2015 was passed and signed by Obama.

The new law says that once the administration submits this deal to Congress, Obama can't waive sanctions on Iran for 60 days so Congress can have time to review the agreement.

However, the burden is then on Congress to disapprove the deal. If Congress takes no action, the lifting of sanctions will go into effect.

Furthermore, a mere majority vote to kill the deal isn't enough. To go into effect and block Obama from lifting the sanctions permanently, Congress's resolution of disapproval would have to overcome Obama's promised veto. For that, the GOP would need two-thirds of both the House and Senate — 290 votes in the lower chamber, plus 67 in the upper one.

In the House, there are currently 246 sitting Republicans — so if every one of them opposed the deal, the remaining 44 votes would have to come from Democrats. The vacant seat of former Rep. Aaron Schock (R-IL) will be filled in a September 10 special election, and Republican Darin LaHood is expected to win it. So if he's seated in time for the vote, only 43 Democrats would need to disapprove the deal.

In addition to that, the Senate would have to vote to override a veto too. In the Senate, there are 54 sitting Republicans, meaning that 13 Democrats would have to join them.

The politics of the Iran deal on the Hill

What this means in practice, though, is that a vote will take place. And that will be an uncomfortable vote for many Democrats. Some may be genuinely skeptical of the deal, and others may be caught between their desire to support the president and their desire to have good relations with hawkish pro-Israel groups like AIPAC (which released a "deeply concerned" statement about the deal Tuesday morning).

But AIPAC has had less clout among Democrats recently. Last year, the Daily Beast's Eli Lake reported on how, in somewhat of a precursor to this fight, AIPAC "helped turn what was a bipartisan effort to keep Iran in check into just another political squabble," when its effort to get Congress to further toughen sanctions on the country failed.

Plus, in addition to President Obama, Hillary Clinton — one of the more hawkish leaders in the party — is praising the deal. That means that Democrats who defect would be going against both the current president and his potential successor.

So, barring a true outpouring of anti-deal opinion and pressure from constituents over the summer, not too many Democrats are likely to defect unless they have cover from some respected figures in the party. Those could include Sen. Chuck Schumer (D-NY) and House Minority Leader Steny Hoyer, who have hawkish foreign policy leanings and have tended to be strong allies of pro-Israel groups. Indeed, earlier this year, it was Schumer whose intervention was key in passing the law that gave Congress this review power in the first place, and he is Senate Democrats' choice to replace Harry Reid as their leader after he retires. So for signs of whether Congress will kill the deal, look to Schumer and Hoyer.