When Congress returns to Washington later this month to tackle a budget impasse and a massive defense policy bill, there will be one more contentious item on its agenda: extending sanctions on Iran.

At the end of the year, the Iran Sanctions Act (ISA) expires — and with it, the regime of existing U.S. sanctions lawmakers say are essential to ensure Washington can “snap back” punitive measures against Tehran should Iranian leaders violate the terms of the nuclear deal that went into effect earlier this year.

But if lawmakers go too far in their bid to renew and possibly stiffen the sanctions, the White House fears Tehran could interpret it as a U.S. violation of the deal — and take that as a cue to fire up their nuclear reactors again.

Few issues of foreign policy have divided Congress as bitterly as the Iran deal.