Their nations are estranged by decades of distrust -- but Barack Obama and Hassan Rouhani share a common political problem.

Hawks and hardliners in Washington and Tehran are putting the squeeze on the US and Iranian presidents as they grasp for a landmark nuclear deal.

As new international talks on Iran's nuclear program open in Geneva, deep mutual suspicion leaves American and Iranian negotiators little time or political space to act.

In Washington, national security conservatives who view Iran's diplomatic charm offensive through the same skeptical lens as Israeli Prime Minister Benjamin Netanyahu, want the White House to swear off inducements for Iranian concessions and threaten to toughen sanctions.

In Tehran, clerical and political conservatives, temporarily silenced by Rouhani's election, disdain talks with the "Great Satan" America.

"It is entirely possible that there are political considerations on either side that will complicate this," said Suzanne Maloney of the Brookings Institution.

"Both sides have to insulate against that to the extent that it is possible."

For Obama, a deal to disable Tehran's nuclear program without using military force would offer a shining foreign policy legacy.

But Democratic and Republican hawks steeped in decades of antipathy towards revolutionary Iran doubt a genuine deal is possible.

A new Senate push for even tougher sanctions than those currently hobbling Iran's economy prompted intense lobbying from a White House fearful that hardliners in Tehran could torpedo Rouhani's diplomatic outreach.

But powerful Democratic Senate Foreign Relations Chairman Robert Menendez and Republican colleague Mark Kirk argue Iran only came to the table because of sanctions -- and urge more tough medicine.

Convincing the Senate to hold off on new measures to further choke Iran's oil sector is one thing -- selling a divided, dysfunctional Congress on interim and final deals brokered by the US team in Geneva is another.

Although Obama has the power to waive certain sanctions on the Islamic Republic, he would need help from lawmakers to offer the full relief that might motivate Iran to do a deal.

But his sway on Capitol Hill is waning amid criticism over his handling of the Syria crisis and a second term malaise.

No 'perfect' deal

The likely shape of any final deal will prove politically contentious.

Menendez is on record with a bipartisan group of senators as saying Iran must be barred, among other restrictions, from enriching uranium -- in line with Israel's red lines on a nuclear deal.

But former Obama administration non-proliferation official Robert Einhorn says a "perfect" deal may already be out of reach.

"The problem is that Iran is exceedingly unlikely to cave in to demands that it regards as tantamount to surrender," he said, in a speech in Tel Aviv last month widely interpreted as hinting at administration thinking.

"Most observers who follow Iran and its internal dynamics believe that no matter how devastating the sanctions, no matter how persistent we are at the negotiating table, and no matter how credible the military option we are able to threaten, Iran will not agree to the maximalist terms that the Israeli government and some Americans advocate."

If Einhorn is right, Obama will have to convince skeptical lawmakers that a less than ideal agreement -- which leaves Iran short of "breakout" capacity to building a nuclear bomb but does not deprive it of the "right" to enrich uranium, is worth supporting.

Diplomatic dance

Rouhani's limited room for political maneuver was reflected in his diplomatic dance at the United Nations in September -- when he dodged a meeting with Obama but spoke to him by phone.

Many Iran watchers here interpret Iran's desire for quick results in the talks as a sign Rouhani is on borrowed time, after winning election in June on a pledge to ease US sanctions.

They also noted Supreme Leader Ayatollah Ali Khamenei's comment Sunday that talking would do Iran no harm.

But Khamenei, with ultimate authority over the nuclear program, also warned he was "not optimistic."

"The reason why the Supreme Leader came out two days ago in support of the negotiating team was to silence some of those opponents who were talking about a sellout," said Haleh Esfandiari, director of the Middle East program at the Woodrow Wilson International Center for Scholars.

"So far he has the support of the Supreme Leader. How long that will last we don't know," she said at a packed Center for Strategic and International Studies forum which reflected high interest in US-Iran gambit in Washington.

US officials are privately satisfied with the makeup of the Iranian negotiating team under Foreign Minister Mohammad Javad Zarif, who experienced US-style politics close up during law studies in Denver and as UN ambassador.

But while there may be some tactical advantage derived from a "good cop, bad cop" routine with lawmakers, there is concern that hawks in Tehran might interpret tough talk in Congress as a provocation.

"Elites in Iran who oppose improved US relations will certainly and understandably read any movement towards sanctions in the middle of negotiations as hostile," said Jim Walsh, an Iran specialist with the Massachusetts Institute of Technology.

Iran's tortuous nuclear standoff with the West
Geneva (AFP) Nov 07, 2013 - Iran's nuclear stand-off with the West has dragged on for a decade, but there are glimmers of hope after Iran presented a closely-guarded proposal at talks in Geneva last month.

That paved the way for a fresh meeting in the Swiss city on Thursday and Friday.

Here is an overview of the issues:

INTERNATIONAL DEMANDS

The P5+1 group -- UN Security Council permanent members the United States, Britain, France, China and Russia plus non-member Germany -- has pushed for concessions from Iran amid suspicion that it is developing nuclear weapon capability, an allegation Tehran denies. The demanded concessions are:

-- To dismantle the Fordo underground uranium enrichment facility, which has nearly 3,000 centrifuges installed and is dug deep into a mountain near the holy city of Qom, some 150 kilometres (90 miles) south of Tehran.

-- To suspend uranium enrichment. Successive UN resolutions call for an end to all uranium enrichment, but the P5+1 is understood to be willing to accept a compromise under which Iran would cease its 20-percent uranium enrichment, which international powers consider dangerously close to a weapons-grade capability. Under that model, enrichment of up to five percent would be permitted for civilian nuclear purposes, though stocks would be limited.

-- To treat the country's remaining 20-percent enriched uranium and change it into combustible material, which then would require weeks of processing to convert back into gaseous form able to be enriched further.

Lead US negotiator Wendy Sherman last weekend defined "progress" as stopping the nuclear programme from advancing further while negotiators try to reach a comprehensive agreement.

She said Washington was prepared to offer "very limited, temporary, reversible sanctions relief" while maintaining the "fundamental architecture of the oil and banking sanctions -- which we will need for a comprehensive agreement, not for a first step".

IRAN'S STANCE

Under Iran's former president, hardliner Mahmoud Ahmadinejad, talks with the P5+1 in Kazakhstan in February and May hit the wall as Tehran rejected the world powers' demands outright.

But the tone has changed since reputed moderate Hassan Rouhani won office in June, pledging to resolve the nuclear dispute and lift international sanctions through constructive engagement.

The proposal Rouhani's negotiators put on the table has been kept secret. But according to Iranian officials, it envisages a first and a last step which Tehran hopes can be implemented within three months and a year respectively.

Iran argues that it will need to see a gradual easing of sanctions along the way, given that both sides should give ground, but officials have said Fordo is a "red line".

The site, built in secret and whose existence was revealed in 2009, began in late 2011 to enrich uranium to 20 percent, most of the way in a technical process to the 90-percent level needed for a nuclear weapon.

Iran says it is enriching to this level to provide fuel for the Tehran Research Reactor, which produces medical isotopes, and denies seeking or ever having sought nuclear weapons.

TIGHTER INSPECTION

The P5+1 are also seeking a tighter inspection regime under which officials from the UN nuclear agency IAEA can have quicker access to all Iran's declared nuclear sites. They also want to be able to go to non-nuclear facilities suspected of being associated with the nuclear programme, such as a military base in Parchin thought to have hosted nuclear warhead design experments, and to access a heavy-water plant at Arak that was formally opened in 2006.

Currently, Iran is only obliged to inform the IAEA three months ahead of transferring fissile material into the nuclear site.

THREAT OF NEW SANCTIONS

Draft legislation for new American sanctions targeting Iran's automobile sector and foreign reserves was adopted in July by the US House of Representatives.

But the US Senate said it would freeze the implementation of the sanctions if Tehran immediately halted uranium enrichment. The White House is trying to persuade them to hold off new sanctions, at least for now.

Israel, the Middle East's sole if undeclared nuclear-armed power, has warned against a partial deal which could see the easing of sanctions, and refuses to rule out military strikes against Iran.

It wants Iran to meet four conditions before sanctions are eased: halt enrichment, remove all enriched uranium from its territory, close Fordo and stop construction of a plutonium reactor.