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Could Congress derail the Iran nuclear deal?

Senate Republicans are pushing legislation that would let them sign off on a nuclear agreement with Iran. And some Democrats may support it.

Congressional Republicans have spent much of the last six years stymieing President Obama’s agenda. But now they could be poised to frustrate the president on perhaps the most consequential issue yet: the proposed nuclear deal with Iran.

And if they succeed, it’ll be because this time they got help from Democrats, too.

The stakes could hardly be higher. The agreement, whose framework was announced by the administration last week after months of intense negotiations, would constrain Iran’s nuclear program, in exchange for relief from crippling economic sanctions. It would immediately form a centerpiece of Obama’s presidential legacy, and, its proponents say, significantly advance the cause of international peace and security.

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Because the tentative pact—which negotiators are aiming to finalize by June 30—isn’t a treaty, Congress doesn’t currently have a role in approving it. But that could change. Sen. Bob Corker (R-Tenn.), chairman of the powerful Senate Foreign Relations Committee, has been building support for a bill that would force Obama to let lawmakers approve any Iran deal.

Corker, who’s known as a deal-maker, is framing his measure as a help, not a hindrance, to the Iran talks, arguing that it strengthens the hand of U.S. negotiators for everyone to know that Congress will have to sign off. Nonetheless, the White House opposes Corker’s bill, arguing that it’s important to maintain the president’s prerogative to negotiate international agreements without interference.

But there’s a real chance that enough hawkish Democrats could sign on to it to override a presidential veto. Assuming all 54 Republicans support it, 13 Democratic votes would be needed to reach the 67-vote threshold.

Fifteen Senate Democrats have already said they could potentially back the legislation, according to Politico’s tally. That includes Sen. Chuck Schumer (D-N.Y.), who is set to take over as the Senate’s top Democrat after Sen. Harry Reid (D-Nev.) retires next year. Schumer, who wields considerable clout in the caucus, had quietly signed on to the legislation weeks earlier. But by coming out more vocally for it, he may free up more of his Democratic colleagues to join him.

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The White House is scrambling to stop Senate Democrats from signing on, or, failing that, to come up with a compromise. But that appears elusive. Obama told The New York Times he might be open to legislation that let Congress “express itself” on the issue in a non-binding fashion. But Corker has said he’s not interested in a non-binding vote.

Senate Democrats are also said to be working with Corker to try to modify the bill. They’re led by Sen. Ben Cardin (D-Md.), who became the top Democrat on the Foreign Relations Committee this week after Sen. Robert Menendez stepped down from that post amid corruption charges.

Of course, Congress isn’t the only potential obstacle to an agreement. Iranian negotiators could face challenges getting sign off from the country’s Supreme Leader, Ayatollah Ali Khamenei. And skepticism from the Israeli government could yet complicate matters.