Iran on Sunday signed an agreement to suspend some nuclear activities in exchange for substantial relief from economic sanctions. The accord was hailed by President Barack Obama as advancing “our goal of preventing Iran from obtaining a nuclear weapon.”

Secretary of State John Kerry’s effort to negotiate a peaceful resolution of Iran’s illegal nuclear program has made promising initial progress. But the Obama administration is making a mistake in opposing efforts backed by a majority of U.S. senators to create a stronger bargaining hand for the United States in the form of sanctions that could be applied immediately if Iran fails to deliver.

Sunday’s agreement is only a preliminary step to serious negotiations to stop Iran’s pursuit of nuclear weapons. American experts put the odds of success in the forthcoming negotiations as no better than 50-50, and some red flags in the interim agreement suggest they could be even more unfavorable.

While the agreement requires Iran to dispose of a stockpile of enriched uranium, and to suspend further production, it will be able to continue work on the Arak heavy water reactor that, when finished, will be capable of producing weapons-grade plutonium. And it can continue work on developing improved centrifuges that will allow it to enrich uranium more quickly.

Although Iran is allowing enhanced international inspection of most of its uranium enrichment facilities, it will not allow international inspection of military facilities where nuclear research has been conducted.

The interim agreement will allow Iran to draw on frozen funds from the sale of oil, and it suspends the ban on Iran’s trade in petrochemicals, autos and precious metals. Additional sanctions will be relaxed as negotiations proceed. Iran has already benefited from the nuclear talks by a recovery of its currency on the world market.

If the interim agreement becomes a framework for the final agreement, Iran will dangerously retain the physical infrastructure to enrich uranium to weapons grade and produce plutonium.

It follows that negotiators for the United States, Britain, France, China, Russia and Germany will need a strong hand if they are to get further Iranian concessions.

Given the high stakes involved, it makes sense to follow the advice of the Senate Foreign Relations Committee Chairman Robert Menendez, D-N.J., and take out “diplomatic insurance” in the form of new stringent sanctions that would be rapidly applied if Iran fails to accept meaningful constraints on its nuclear program. His bipartisan bill to do so is supported by a strong majority of the Senate.

Not only has the president threatened to veto such legislation if it passes, his staff has foolishly maligned Democratic and Republican senators supporting the Menendez measure as warmongers who want “the United States to take military action” against Iran.

Such stubbornness and reckless rhetoric will only alienate the president from legislators who share his desire for a successful treaty, and weaken his bargaining hand against Iran.