Molly Riley/AP

Whether the issue is immigration or healthcare, Congress and the Obama administration have battled as much in court as they have on the House or Senate floor. Another clash over the balance of powers is looming, this time over which branch of government should have the final say on a possible nuclear agreement with Iran.

The Senate is nearing a veto-proof majority on legislation that would require the administration to submit any final deal with Iran to Congress, which would then have 60 days to review and possibly reject it. The White House opposes the bill, in keeping with its longstanding position that Capitol Hill should more or less butt out of the Iran talks. (Knowing that any deal would never earn support from the 67 senators it takes to ratify a treaty, the administration deliberately structured the Iran accord as an "executive agreement" instead.)

The key difference between the Iran fight and the other skirmishes that the White House has had with Republicans in Congress is that, as Peter Beinart and others have noted, President Obama is missing the support of a surprising number of his traditional Democratic allies. The legislation authored by Senators Bob Corker and the recently-indicted Robert Menendez now has eight Democratic co-sponsors, including the minority-leader-in-waiting, Chuck Schumer. That puts it just a few votes shy of the two-thirds majority it takes to override a veto. A similar dynamic could play out in the House, where Nancy Pelosi, the Democratic leader, will try to limit defections against the president. The administration has succeeding in holding Congress at bay for months, but the Foreign Relations Committee, which Corker leads, is set to consider the proposal next week.

As supporters of the Corker-Menendez bill have pointed out, passage of their legislation would not necessarily kill the Iran deal, nor would it explicitly require congressional approval. Lawmakers would have the option of approving or disapproving the accord, but it would go into effect after 60 days if Congress took no action. The administration, however, has argued that the mere hint of formal congressional interference could scuttle the negotiations. And if Congress did force its involvement with the Corker-Menendez legislation, it would mark the first time in 30 years that Congress has overridden a presidential veto on a matter of foreign policy, noted James Lindsay, senior vice president at the Council on Foreign Relations. In 1985, lawmakers defied President Reagan to impose sanctions on South Africa's apartheid regime.

Would the Obama administration have to abide by the Corker-Menendez bill, or could it argue that it infringes on the president's expansive power to conduct foreign policy? "There's a gray area regarding how much inherent authority the president has to enter into agreements with foreign countries," said Michael Dorf, a professor of constitutional law at Cornell University. That question might not be tested unless Congress took the next step of actually voting to disapprove a deal the administration struck with Iran—a resolution that itself could be subject to a presidential veto. That might be a stretch, as even some Democrats who support the Corker-Menendez bill as a matter of principle might ultimately support the final agreement that the administration reaches. While previous disputes over Obama's moves to circumvent Congress in implementing healthcare and immigration policy have resulted in lawsuits, the Iran fight might not get that far, and Dorf said "there's less of a chance of judicial intervention" in a foreign policy matter.

Central to the debate over Congress's role in the Iran negotiations is the fact that any agreement would involve lifting sanctions that Congress imposed in the first place. (Some of the sanctions were put in place by the UN or by executive order.) If Tehran upholds its end of the deal, the White House has said it would suspend congressionally-imposed sanctions by invoking waivers in the law. But that wouldn't erase them permanently. "The reality is that U.S. sanctions aren't going to be lifted until Congress chooses to lift them," Lindsay said. "The president's ability to relax sanctions on Iran is quite limited."

All of this, of course, presumes that the U.S., Iran, and European nations will finalize an agreement in the next three months. That remains a dicey proposition, particularly in light of a series of tweets sent out Thursday morning by Iran's supreme leader warning that Americans "always deceive and breach promises." Yet it's clear that however much the president wants to keep Capitol Hill from scuttling his tentative agreement, Congress will eventually have its say, one way or another.

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