NUCLEAR NEGOTIATIONS with Iran are proving as difficult as skeptics expected following a preliminary agreement in November. Though President Obama announced the accord in a national television address on Nov. 23, only this weekend was a plan for implementing the interim agreement finalized. In the meantime, Iranian negotiators staged one walkout, continued to stonewall a U.N. investigation of past work on nuclear weapons and polished designs for new centrifuges. Israeli warnings that the deal could cause remaining sanctions to crumble look worryingly prescient: Russia is reportedly negotiating a major oil-for-goods deal, and French business owners are booking trips to Tehran.

Implementation of the interim agreement should stop Iran’s highest-level enrichment of uranium, reduce its stockpiles of that material by half and prevent the installation of additional centrifuges and the start up of a new reactor capable of producing plutonium. However, as Iran’s chief negotiator put it Monday in describing the terms of the deal, “No facility will be closed; enrichment will continue, and qualitative and nuclear research will be expanded.” Even if the sanctions regime remains intact, Iran will benefit from a relaxation the Obama administration says is worth about $7 billion.

Pursuing negotiations on these terms, though risky, is preferable to unrestrained Iranian enrichment and a slide toward war. Still, it’s understandable that many in Congress are worried that Iran will escape from the economic corner that sanctions have put it in without meaningfully reducing its nuclear infrastructure or its potential to build a bomb. Those concerns have prompted 59 senators, including 16 Democrats, to back a bill sponsored by Sen. Foreign Relations Committee Chairman Robert Menendez (D-N.J.) and Republican Mark Kirk (Ill.) that would mandate sweeping new sanctions if no final accord is reached or if Tehran violates the existing deal.

In a commentary in The Post last week, Mr. Menendez called his bill a “diplomatic insurance policy” that by “spell[ing] out precisely the consequences should agreement fail” should “motivate Iranians to negotiate honestly and seriously.” There is an obvious logic to this: After all, it was tough sanctions passed by Congress in 2010, resisted by the Obama administration, that helped bring about Iran’s recent change of government and its leaders’ willingness to negotiate.

The White House’s strident opposition to the bill continues a distasteful practice — also seen in the debate over Syria — of accusing all who disagree with its strategy of favoring war. That’s an insult to serious legislators such as Sens. Benjamin Cardin (D-Md.) and Mark Warner (D-Va.), both of whom reasonably seek to hedge against failed negotiations.

Nevertheless, the senators already may have accomplished the maximum good by proposing the bill, thereby raising the pressure on the administration and Iran. Passing it — which probably would require overcoming a presidential veto — would be problematic. Among other things, the bill inserts Congress into the negotiations by spelling out provisions that must be included in a final deal and linking the new sanctions to terrorism, Iran’s missile tests and a nuclear settlement.

This diplomacy has been President Obama’s initiative, and he should be allowed to carry it through to success or failure. Congress then would have a mandate to act, either in lifting sanctions or in redoubling them.