THE DIPLOMATIC betting is that the negotiations on Iran’s nuclear program will end with an agreement to defer Monday’s deadline for a few months. Sanctions on Iran would remain, as would Tehran’s suspension of its most dangerous nuclear activities. While the result may displease many in Congress and in the Iranian regime, it would be the best outcome.

An extension of the status quo will prevent Iran from triggering a crisis by ramping up uranium enrichment. Meanwhile, pressure will grow for it to make concessions it currently is resisting. That’s because even without a further tightening of sanctions, the collapse of global oil prices will strain an economy that has reportedly recovered only slightly in the year since the interim agreement was struck. Iran’s budget depends on an oil price of $140 a barrel, but prices have fallen to below $80, and many experts believe they will remain there in 2015.

In Washington, delay is also useful because President Obama has failed to convince congressional leaders of both parties that the settlement terms he has reportedly offered are sensible. The administration and its allies in the talks reportedly are prepared to allow Iran to preserve a substantial part of its nuclear infrastructure — including 4,000 or more centrifuges. A deal would also be time-limited, meaning all restrictions on Iran’s program would lapse on an agreed date.

While Mr. Obama may believe there are sound reasons for completing a deal on such terms, the White House has been showered with letters from Democrats and Republicans warning against any accord that leaves a substantial nuclear infrastructure in place or that expires in less than decades. Legislators are also rightly concerned about Iran’s failure to cooperate with an investigation by international inspectors on its suspected previous work on nuclear warhead designs, or to accept limits on its development of long-range missiles.

Though Mr. Obama could try to sidestep congressional disapproval of an agreement and use waiver authorities to unilaterally lift sanctions embedded in law, that would be an improper and probably unsustainable way to handle a grave matter of national security. As a letter signed by 43 Republican senators put it, “Unless the White House genuinely engages with Congress, we see no way that any agreement consisting of your administration’s current proposals to Iran will endure in the [incoming] Congress and after your presidential term ends.”

Major U.S. allies in the Middle East, including Israel, Saudi Arabia and other Persian Gulf states have similar concerns, exacerbated by indications that Mr. Obama is willing to accommodate Iranian interests in Iraq, Syria and Lebanon. Arab officials are saying that a deal that allows Iran to preserve a nuclear infrastructure, and creates a deadline for removing all limits on it, will prompt a race by rival states to match that capacity.

The United States should be seeking to weaken and roll back Iran’s influence in the Middle East and to eliminate — not temporarily freeze — its capacity to build a nuclear arsenal. The agreement the administration appears to be contemplating could solidify Iran’s power while setting up another confrontation with Congress and allies. Better to extend the negotiations — and insist on better terms.