CAIRO - Iran's Islamic regime has survived a devastating war with Iraq, strong American sanctions and international isolation in its 30 years of power. It has seen reformist and hard-line presidents come and go, with barely a flinch.
But now, public anger over the disputed election has given Iran's ruling elite a challenge of a new and unsettling kind: A growing opposition movement with apparent broad backing, headed by a leader who is one of their own - and doesn't seem intimidated.
Iran's clergy-guided system does not appear in immediate danger. But the ruling clerics are paying close attention to the street anger - the same popular unrest they harnessed themselves three decades ago to bring down the shah in their 1979 revolution.
There is a chance - just a chance - that the recent protests could transform into a serious, credible movement similar to an opposition party in another country, fundamentally changing a system now ruled by an all-powerful and untouchable theocracy.
There is also a chance, more likely, that to avoid such an outcome, the clerics will either jettison, or at minimum rein in and weaken, the president they have supported until now, hard-liner Mahmoud Ahmadinejad.
For that to happen, the protests would almost certainly need to be sustained, spread to other cities and most importantly, attract enough clerical support to create high-level rifts.
"No one is very sure where it will go next," said Suzanne Maloney, an Iran expert at the Saban Center for Middle East Policy in Washington. But she says: "They have forced the regime to take a step back."
Despite that, the likelihood that Iran's clergy-ruled system will undergo a radical change remains dim.
Many Iranians feel strong kinship with the revolution, its heirs and the system they created, and are reluctant to do anything that would trigger bloody upheaval again. Ahmadinejad has broad support among the poor and pious, who also venerate the country's supreme leader, Ayatollah Ali Khamenei.
Yet Ahmadinejad's hard-line jousts with the West, his mishandling of the economy and, in particular, what many view as a blatant theft of the election, seem to be turning off growing segments of the middle class.
The recent protests are different from the country's last unrest, student-led protests in 1999 that fizzled. In particular, this go-round has attracted some of Iran's middle class, the same group that changed a religious movement in 1979 into a strong revolutionary force.
The leader of the street protesters this time, rival presidential candidate Mir Hossein Mousavi, is also different from the reformist student leaders of 1999. He is no sideline player or amateur but an experienced politician who was prime minister in the 1980s during Iran's tough war with Iraq, when Khamenei was president.
Mousavi does not appear intimidated by the supreme leader or his inner circle. Indeed, he can take his complaints right to them, and he can make their life rough if he begins to criticize the clerical system as complicit in protecting Ahmadinejad.
There's no way to know if Mousavi will challenge, or would even want to challenge, the Islamic system itself. He is a product of the revolution, never known as a reformer in the past. Yet he has already gone further than many expected.
The country's clerics may hold the final word.
Iran's power structure has always been opaque. Essentially it consists of a broad base of clerics supporting a ruling elite of high-level clerics, who have the power through various institutions to overturn the decisions any president makes.
At their top is the country's supreme leader - Khamenei, who controls the armed forces, other security forces and the nuclear program. He serves as final arbiter.
But even Khamenei must be careful lest he lose the support of the clerics who empower him. A rift in the high levels of the clerical structure could endanger even him, the supremacy of his position and the clerical system itself.
Some clerics in key institutions like the Guardian Council - which vets the election - are lockstep backers of Khamenei. But others, such as former President Hashemi Rafsanjani, a fierce critic of both Khamenei and Ahmadinejad, are wild cards.
The concessions that have already come from the clerics, especially the announcement to re-examine the vote tally, show that at least some clerics want to keep the protests from escalating.
So far, Mousavi has made no direct threat to the Islamic system.
But he hinted at it during a massive rally of supporters Monday, telling them to stand up to "this astonishing charade ... Otherwise, nothing will remain of people's trust in the government and the ruling system."
In Iran's post-revolutionary history, protesters have almost never crossed that red line and demonstrated against the ruling clerics themselves.
If they do, it would leave the clerics in a difficult spot: Choosing between making political concessions to reformists, or watching the unrest grow to ever-more dangerous levels.
Revolutionary change like that is still far away and may never come. But the possible contours of the once-unthinkable are now taking shape.
EDITOR'S NOTE - Sally Buzbee is the Mideast Editor for The Associated Press based in Cairo. Associated Press writer Brian Murphy contributed to this report.
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