Update: this action, now closed, ran in October 2018.
A new bill introduced in the Senate would block President Trump from launching an unnecessary war against Iran. Introduced by Senator Tom Udall and enjoying bipartisan support, the Prevention of Unconstitutional War with Iran Act of 2018 draws on Congressional power of the purse and power to declare war in to constrain the Trump administration. The legislation mandates that no funds may be used for military operations against Iran absent Congressional authorization for such operations or an imminent threat to the United States.
The possibility that the Trump administration may launch an ill-advised American strike against Iran, reminiscent of the 2003 Iraq war, is real and dangerous. Trump has surrounded himself with Iran war hawks like National Security Advisor John Bolton and Secretary of State Mike Pompeo. At their counsel, the United States is now violating the Iran nuclear deal, which increases the likelihood of an unnecessary war. And Trump’s Twitter threats against Iran — for example, warning that Iran will “suffer consequences the likes of which few throughout history have ever suffered before” — increase the risk of miscalculation by both sides.
The text of the legislation is here.