The Obama administration is facing its last best chance to curb Iran's nuclear program — not just to meet an end-of-the-month deadline for a deal, but also to seal one before skeptical Republicans who will control Congress next year are able to scuttle it.

Years of negotiations to limit Tehran's nuclear production entered the final stretch Sunday as U.S. Secretary of State John Kerry met with Iranian Foreign Minister Javad Zarif and European Union senior adviser Catherine Ashton in Oman's capital. With no immediate agreement in sight, officials said the discussions were expected to continue into Monday.

The stakes are high as the Nov. 24 deadline approaches. A deal could quell Mideast fears about Iran's ability to build a nuclear bomb and help revive the Islamic Republic's economy.

It also would deliver a foreign policy triumph for the White House, which is being hammered by prominent Republican senators over its handling of the civil war in Syria and the growth of the Islamic State militancy in Iraq. Those same critics seek to put the brakes on U.S.-Iranian bartering, if not shut it down completely, once they seize the majority on Jan. 3.

The Obama administration "needs to understand that this Iranian regime cares more about trying to weaken America and push us out of the Middle East than cooperating with us," Republican Sens. John McCain of Arizona, the incoming chairman of the Senate Armed Services Committee, and Lindsey Graham of South Carolina, said in a statement last week.

President Barack Obama told CBS' "Face The Nation" that his administration's "unprecedented sanctions" on Iran are what forced Tehran to the negotiating table. "Our number one priority with respect to Iran is making sure they don't get a nuclear weapon," he said.

But Obama also cited "a big gap" between Iran and world powers as they try for a final agreement. "We may not be able to get there," he said in the interview broadcast Sunday.

Over the past year, congressional Republicans have made little secret of their skepticism of Obama's outreach to Tehran. They say it has alienated Israel and kept the U.S. from maintaining a hard line on a number of foreign policy fronts, including Iran's detention of three Americans.

That skepticism is borne mostly of concerns that Iran secretly will enrich enough uranium to build nuclear weapons, even after a deal is reached. For years, Iran hid some of its nuclear facilities and blocked inspectors' access at others, raising widespread alarms about its intentions.

Penalties imposed by the U.S., EU and the U.N. Security Council aimed to punish Tehran for its covert nuclear program.

Iran has maintained that its nuclear activities are purely peaceful and necessary to fuel medical and energy demands.

Last week, Kerry, a former Senate Foreign Relations committee chairman, rejected suggestions that a GOP-controlled Congress would be able to change course on negotiations with Iran. He also noted that any Senate move would need overwhelming support to be approved. "As we have learned in the last few years, the minority has enormous power to stop things from happening," he said.

He also has said none of the world powers has an appetite for extending the talks beyond the Nov. 24 deadline, although that remains a remote possibility if an agreement appears close.

If a deal is struck before year's end, U.S. lawmakers may have limited ability to undo it. Experts believe most of the U.S. penalties against Iran's financial and oil markets can be suspended, if not lifted entirely, by presidential authority.

Beyond Jan. 3, however, and without an agreement in place, Congress could try to issue new sanctions without giving Obama that authority to suspend or lift them. Already, a plan to strengthen them if the negotiations expire without a final deal has gathered strong backing from senators from both parties.

If the two sides are close at that point, the administration almost certainly would move to veto any legislation imposing new penalties, or ones that would otherwise tie Obama's hands. Administration officials believe new sanctions could violate the negotiating terms and lead Iran to step up its production of enriched uranium.

But if the negotiations drag on, the White House will have to decide whether it could accept sanctions with a threatened "trigger" to be enacted in future months.

A senior U.S. official said such triggered sanctions could appease anxious lawmakers, while at the same time push Iran more quickly toward a deal. The U.S. official made clear that triggered sanctions are not currently being considered by the White House. The official was not authorized to discuss the strategy by name and spoke on condition of anonymity.

"If Congress can be convinced that the overall framework for a deal is in place and it really is just loose ends that need tying, then I think it would probably wait until seeing a final deal before taking any action," said James Acton, co-director of the nuclear policy program at the Carnegie Endowment for International Peace in Washington.

"However, it may be hard to persuade Congress that a deal is just around the corner," Acton said.

Broadly, a potential agreement would ease economic sanctions against Tehran agrees to limit its uranium enrichment to a level that would make it unable to build nuclear weapons. It would have to provide international inspectors with full and verifiable access to Iran's nuclear facilities.

Iranian officials appear guardedly optimistic about reaching an agreement by the end of November, but insist on a quick lifting of the sanctions.

Iran agreed to freeze its uranium enrichment during the negotiations and reduce its stockpile of weapons-grade nuclear

"Sanctions have never contributed to the resolution of this issue," Zarif told reporters as he headed to Muscat. "They must be removed. They have not produced any positive results."

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Associated Press writers Ali Akbar Dareini in Tehran, Iran, and Adam Schreck in Dubai, United Arab Emirates, contributed to this report.

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