US President Donald Trump’s comprehensive policy approach to Iran, outlined in a major address to the nation last week, hinges on a legislative proposal in the Senate that has yet to be published.Trump wants lawmakers to amend the Iran Nuclear Agreement Review Act of 2015 – a law that provides Congress with oversight powers over the nuclear agreement with Tehran brokered that same year – to impose unilateral “triggers” that would snap back US sanctions on Iran for behavior not outlined by the agreement itself. The amendment has been in the works for weeks, drafted by Republican Senators Bob Corker of Tennessee and Tom Cotton of Arkansas. These lawmakers hope the new legislation will provide Trump with muscle going into a new round of negotiations with Iran over its nuclear work, where they hope for a renegotiation of the 2015 deal at best and, at minimum, an addendum to it.Corker and Cotton have worked closely on the legislation with the State Department, but they have not yet released its text, making it difficult to gauge support on the Hill for its passage.Secretary of State Rex Tillerson last week told reporters the legislation would include specific triggers on Iran’s nuclear activities, but also on behavior not addressed in the nuclear deal, such as its ballistic missile activity.The key provision of the amendment, according to a press release from the two senators, is “automatic snap back of US sanctions should Iran go under a one-year ‘breakout’ period and move closer to a nuclear weapon.” “These restrictions remain in effect indefinitely, effectively ridding the JCPOA of its sunset clauses as they apply to US sanctions,” the senators said in a joint statement, referring to provisions of the deal applying caps on Iran’s nuclear work that expire over time.Initial reaction from Democrats was swift, and roundly negative, fearing the amendment would undercut US leverage with America’s closest allies. The caucus appears unlikely to support any legislation that will “automate” snapback over the growth of Iran’s nuclear program – much less on non-nuclear behavior, which would be seen as a violation of the deal by several parties to it.It will be a challenge to create consensus on Capitol Hill over how to define a “one-year breakout period” – the amount of time it would take Iran to go from making the political decision to build a bomb to actually having one constructed. This geopolitical term refers to an imprecise formula of a nuclear program’s size and efficiency.Tillerson says he hopes Congress will act within 90 days – before the president is next required to certify the worth and veracity of the nuclear deal under US law.