Among the many revelations in a recently leaked interview with Iranian Foreign Minister Mohammad Javad Zarif is a passing reflection on the nuclear deal his country signed with the world powers in 2015. Zarif admits he was “naive” to assume that U.S. President Barack Obama could keep a bargain he had made without approval from Congress.

He’s right. The Joint Comprehensive Plan of Action, as the deal is formally known, has been a case study in the limitations of presidential power in dealmaking. The absence of any congressional imprimatur on the agreement made it easy for Obama’s successor, Donald Trump, to simply walk away from it.

Now, as the signatories enter the home stretch in negotiations to bring the U.S. back into compliance, they’re making the same mistake. President Biden, keen to resurrect the deal, is exerting no meaningful effort to bring Congress onside; the Iranians, despite questioning the reliability of American promises, are not insisting they be backed by law.

The smarter course for both sides is to hold out for a treaty, sanctioned by the U.S. Senate.

Without such affirmation, the credibility of a resuscitated nuclear deal will remain contingent on the political calculations of presidents to come. In turn, this will undermine the economic value of the deal: Who would invest in Iran in the knowledge that sanctions could easily be reimposed by the next occupant of the White House? And if the dividends of the deal fail to meet Iran’s expectations, it may well feel compelled to reconsider its own commitments.

An agreement that mitigates the menace Iran poses to the Middle East and forestalls conflict with the U.S. is far too important to leave to executive whim. Both Washington and Tehran should make good-faith efforts to achieve domestic consensus.

The Iranians will have an easier time of it. While there is some divergence in views about the deal among different political factions, there is general agreement on the need to free the Islamic Republic from economic sanctions. This is also the view of Supreme Leader Ali Khamenei – and unlike the American president, his office is not subject to the fluctuations of electoral politics.

Biden has a higher mountain to climb. Under the U.S. Constitution, turning the nuclear deal into a treaty would require the approval of two-thirds of the Senate. On the face of it, this seems an impossible standard. There is bipartisan skepticism about the agreement, and broad support for a revised deal that addresses not only Iran’s nuclear threat but also the other ways it endangers the Middle East, including its support for terrorist groups and sectarian militias.

Khamenei has said Iran will not widen the scope of the negotiations currently underway in Vienna, but it was not so long ago that he was refusing even to discuss its nuclear program. The election of a new president this month will give Tehran an opportunity for a reset. Biden needs to convince the Iranians and Congress that a comprehensive treaty is the outcome they both want – a deal that will survive the vagaries of time and presidential politics alike.