An Iranian nuclear agreement is the Obama administration's grandest foreign policy objective, a legacy-defining endeavor that holds the prospect of ending the gravest potential threat to Israel and the Middle East and reintegrating Iran into the world community.

But reaching a deal is no easy matter. And as U.S. Secretary of State John Kerry plunges back into talks Wednesday with Iran's foreign minister, many challenges beset the diplomacy.

Iran is maintaining a tough line on much of the nuclear infrastructure that it says is for peaceful energy production, but which world powers worry may be designed to develop atomic weapons. President Barack Obama's negotiators are offering the Iranians permanent relief from economic sanctions, yet are struggling to convince an unruly U.S. Congress to cooperate.

With impatience rising in both countries, a Nov. 24 deadline for an accord looms. Washington and Tehran each have spoken vaguely about a second extension of the talks in four months. Neither side wants the alternatives: fast-developing Iranian nuclear advances, more crippling U.S. economic pressure and, with Israel vowing to stop Iran by any means, maybe even a new Mideast war.

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HOW DID WE GET HERE?

After more than a decade of stop-and-go negotiations, diplomacy with Iran heated up after last year's election of moderate-leaning President Hassan Rouhani.

Within months of Rouhani taking office, Iran and world powers reached an interim agreement imposing strict limits on Iran's enrichment of uranium and halting work on a heavy water reactor that would potentially produce plutonium. Both materials can be used in nuclear warheads.

In exchange, the U.S. granted Iran eased trading conditions and access to funds frozen in foreign accounts — some $7 billion in combined relief.

The plan was for a permanent deal by late July. But the U.S., its five negotiating partners — Britain, China, France, Germany and Russia — and Iran failed to build on the early promise of the talks. The deadline was pushed back a further four months.

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WHAT MIGHT AN AGREEMENT LOOK LIKE?

Iran has won tacit acceptance of its biggest priority: recognition of its right to enrich uranium.

After years of demanding an end to all such activity, the U.S. and its partners now speak only of limiting the amount of centrifuges Iran can have in operation and the amount of material Iran can stockpile for enrichment. A compromise could be to cut the number of centrifuges in half from their current level of about 9,000. Other technical safeguards also are being considered.

The interim deal's stepped-up monitoring of Iran's nuclear activity would surely be continued, and likely expanded. Negotiators have spoken of creative options for redesigning Iran's heavy water reactor project so it cannot produce plutonium.

The United States has promised to scrap its "nuclear-related" sanctions on Iran in the event of a deal. Those could include global restrictions on Iranian oil, banking and manufactured trade.

A final pact would lock in place the conditions for each side for several years, though the Americans and their partners are pushing for a longer duration than the Iranians.

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WHAT ARE THE REMAINING CHALLENGES?

The devil of any deal is in the details. Fearful the Iranians could inch toward weapons production despite the nuclear restrictions, Washington will be looking for as many safeguards as possible.

Iran needs to know what economic measures the U.S. will lift from an overlapping set of sanctions targeting the nuclear program as well as Iran's human rights record, alleged terror links and development of ballistic missile technology.

Domestic pressures in Tehran and Washington could prove deal-breakers.

Any pact needs the blessing of Iran's supreme leader, Ayatollah Ali Khamenei, whose support has been ambiguous. Other Iranian hardliners have strenuously objected to any concessions.

Obama, meanwhile, needs Congress to agree to permanently scrap any sanctions. Opposition runs deep among Republican and Democratic hawks. And a potential Republican takeover of the Senate in 2015 could make significant U.S. concessions more difficult.

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WHAT ARE THEY SAYING?

Asked about the deadline in Paris on Tuesday, Kerry wouldn't make a prediction. "I don't believe it's out of reach but we have some tough issues to resolve," he said.

One of his key partners, Russian Foreign Minister Sergei Lavrov, was more optimistic. "I'm sure a compromise is possible," he said. Still, he said the deadline was "not sacred."

Iran's Rouhani said this week he believed a deal "can be achieved."