Coons' proposal that Trump issue a signing statement amounts to an intervention, on the side of Mattis and other advisors, in an internal administration debate about how best to convince Iran and Europe to start negotiating new restrictions on Iran's missile program and other aggression in the region.

"There are no big fans of the deal within the administration," said one expert who advises the administration, on condition of anonymity. "The main divisions are, really, what do we do to try to fix the deal?"

The president's preference for declaring the deal inconsistent with U.S. interests has some prominent congressional allies, however. Sen. Tom Cotton, R-Ark., has argued that he should refuse to certify the deal, but then delay walking away from the pact entirely. That would give time to negotiate new terms to the agreement to counter various forms of Iranian aggression, according to the Arkansas Republican.

Coons worries that Congress will come under pressure to impose sanctions after Trump declares the deal contrary to American interests. "The president could successfully stir up action in Congress that would blow up this deal, which I do not understand to be his policy goal, but may end up being the political outcome," he said. "The result would be that Congress would then be responsible for blowing up the deal, not the president."