BEIRUT (Reuters) - Iran's post-election power struggle is shaking the Islamic Republic to its roots, with no sign that its supreme leader can assuage popular anger or regain the trust of alienated politicians and clerics any time soon.
The turmoil since hardline President Mahmoud Ahmadinejad was re-elected in a June 12 vote his opponents said was rigged has rendered moot U.S. President Barack Obama's offer of engagement with Iran, which the West suspects of seeking nuclear weapons.
It is hard to see how Iran could forge consensus on nuclear negotiations or dialogue with the United States while Supreme Leader Ayatollah Ali Khamenei is facing unprecedented challenges to his authority and deep fissures within the ruling elite.
Tehran, which says its nuclear program is purely peaceful, faces a September deadline to agree to substantive negotiations with the West or risk tougher international sanctions.
"Whether or not you can find an interlocutor in Iran at this moment is questionable because the whole regime is in crisis," said Rasool Nafisi, a U.S.-based Iran expert. "Whatever move it made would be perceived as either arrogance or weakness."
The nuclear issue may have to await the outcome of Iran's gravest internal upheaval since the 1979 Islamic revolution.
A hard-hitting Friday sermon by Akbar Hashemi Rafsanjani, a disaffected regime heavyweight, has re-energized the opposition after security forces quelled last month's huge street protests and jailed hundreds of prominent reformists and intellectuals.
"The main problem the opposition faces is that their brains trust is either in prison, under house arrest, or unable to communicate freely," said Karim Sadjadpour, an analyst at the Washington-based Carnegie Endowment for International Peace.
"There remains tremendous popular outrage, but at the moment there is no leadership to channel that outrage politically."
Sadjadpour saw no sign of an early compromise because hardliners feared any concession would only encourage their foes, and for now they still control the levers of power.
"While there are pronounced cleavages among Iran's clerical elite, Khamenei's power base is not the clergy but the Revolutionary Guards. When and if we start to see rifts among the Guards it could be fatal for both Khamenei and Ahmadinejad."
Khamenei, apparently stung by Rafsanjani's sermon in which he said Iran was in crisis because of doubts over the election result and demanded an end to detentions and press curbs, warned senior figures on Monday not to help Tehran's enemies.
"Elites should know that any talk, action or analysis that helps (the enemy) is a move against the nation," he said.
But Rafsanjani appears too powerfully entrenched to ignore. "He is not just anybody. He is one of the leading figures of the revolution and well-anchored among large groups of clerics," said a Western diplomat in Tehran. "He has a fantastic network."
Defeated candidates Mirhossein Mousavi and Mehdi Karoubi have defied Khamenei's efforts to silence their protests. Former President Mohammad Khatami, a mild reformist, on Monday boldly proposed a referendum on the legitimacy of the government.
Ideologically, the backing of Iran's clerical establishment is crucial for the leadership's standing, but few ayatollahs have endorsed Ahmadinejad and some, like dissident Grand Ayatollah Hossein Ali Montazeri, have even attacked Khamenei.
"The clerics don't know what to do," said Baqer Moin, a London-based Iran analyst. "They are stuck between Khamenei's uncompromising position and an emboldened opposition which is demanding more than Khamenei is willing to give."
Khamenei, who succeeded revolutionary founder Ayatollah Ruhollah Khomeini as supreme leader in 1989, has endorsed Ahmadinejad and the election result, risking his own credibility as a mediator in disputes and ultimate arbiter of state policy.
"The supreme leader has lost his power to pronounce the final word," said Moin, a biographer of Khomeini. "He is just another politician ... reflecting the interest of various groups surrounding him, not the system as a whole."
Rafsanjani, 75, a veteran insider who heads a body that can in theory dismiss the supreme leader, was accused of corruption by Ahmadinejad during a mud-slinging election campaign.
But he took a lofty line at Friday prayers, saying unity and people's trust in the voting process must be restored after a "bitter day" -- trenchant indirect criticism of Khamenei.
"Rafsanjani has played it quite cleverly and in fact gave the speech the leader should have," Ali Ansari, an Iran scholar at Britain's Durham University said of Friday's sermon.
At stake now, he said, was the political survival not just of Ahmadinejad, but of Khamenei, who was "weakening by the day."
Some analysts see an outside chance that Iran's hardline leaders, feeling isolated at home, might seek an accommodation with the West to gain legitimacy and shore up their position.
So far, however, Khamenei and Ahmadinejad have repeatedly accused Western powers of inciting post-election unrest and have ruled out any concessions on Iran's nuclear projects.
Conversely, Obama may be in no haste to embark on any negotiation that might bolster Iranian leaders caught up in such a fluid, unpredictable political drama in Tehran.
"I fear Ahmadinejad's presence serves as an insurmountable obstacle to confidence-building with the United States," Carnegie's Sadjadpour said of the prospects for engagement.
"It's going to be impossible for Tehran to reassure us that its nuclear ambitions are purely peaceful as long as Ahmadinejad remains so outspokenly belligerent toward Israel."
Editing by Jon Hemming