This week the Iran nuclear fight shifts to Congress, as the Corker-Menendez bill goes before committee and likely to the floor. The bill would allow Congress to block the implementation of an Iranian nuclear deal, but would have to clear several significant hurdles before then. The first hurdle is the lowest, but how the bill handles it will tell us a lot about its ability to clear the others.On Tuesday, the bill will come before the Senate Foreign Relations Committee, which consists of ten Republicans and nine Democrats (Senator Corker, R-TN, is the Chairman). Two of the Committee’s Democratic members, Senator Menendez and Senator Tim Kaine, are sponsors of the deal, so it will likely pass on either Tuesday or Wednesday with a minimum of 12 votes. If the tally is higher, as sources anticipate, that would be a good sign for the bill’s overall prospects. (The bill will need to secure sixty votes in the Senate to avoid a filibuster—likely, as it has 1 Independent and 8 Democratic sponsors—and 67 votes to override the President’s threatened veto.)In order to secure wider Democratic backing, Corker has opened the bill to amendments. Some of these would be fairly innocuous—measures such as shortening from sixty days to thirty the period during which the law would freeze the President’s ability to suspend sanctions on Iran in the wake of a final agreement, to give Congress a chance to vote to disapprove of it. Some would give the GOP’s right wing valuable talking points, such as decoupling the sanctions from Iran’s continued promotion of terrorism (a point Senator Tom Cotton made in an interview with Jeffrey Goldberg in the Atlantic on Monday), but might also make the bill more realistic under current circumstances. And others would serve as “poison pills”, likely ruining the bill’s chances. Among the latter is a provision that would allow the President to suspend sanctions until Congress had voted—which would allow him, bill supporters argue, to present the end of the sanctions regime as a fait accompli.Corker and his supporters are navigating tricky waters, trying to maintain a bill strong enough to keep hawks on board but not so rigorous as to cost it the support of Democrats wary of straight-jacketing the President. Complications include not only the tensions in managing such an ideologically broad coalition, but also the fact that the Committee has a new Democratic ranking member, Senator Ben Cardin of Maryland, since Menendez had to step aside in the wake of a corruption indictment.Corker-Menendez lies at the heart of the most significant Executive-Legislature power struggle in decades. It is in a strong, but by no means certain, position. It may get off the ground and achieve escape velocity, or else explode on the launch pad. As things start tomorrow, keep an eye out for signs pointing in either direction.