WASHINGTON—The mounting popular demand that Egypt's all-powerful ruler step aside has suddenly forced the United States to contemplate a Mideast without the guarantee of a bedrock Arab ally -- and raised the specter of the anti-American revolution in Iran a generation ago.

The spiraling turmoil in Egypt is confronting the Obama administration with its most acute foreign policy crisis to date, and officials are toeing a delicate line.

There are appeals to 82-year-old Egyptian President Hosni Mubarak for restraint and reform alongside signals to the protesters that the United States understands their frustration and supports at least some of their goals. In equal measure, the Obama administration is trying to avoid unintentionally aiding a militant Islamic takeover if Mubarak falls.

President Barack Obama called the leaders of Israel, Turkey and other nations worried about what comes next in Egypt, the most powerful Arab guarantor of Israel's security for more than 30 years.

"The president reiterated his focus on opposing violence and calling for restraint; supporting universal rights, including the right to peaceful assembly, association, and speech; and supporting an orderly transition to a government that is responsive to the aspirations of the Egyptian people," according to a White House describing the calls.

Secretary of State Hillary Rodham Clinton used similar language Sunday about an "orderly transition" to a responsive government. It's the closest Washington has come to hinting that Mubarak may be through.

The cautious U.S. holding pattern over the past several days underscores deep concerns that developments in Egypt will reverberate throughout the Arab and Muslim world from Morocco to Malaysia.

In the immediate area, Iran, Iraq, Lebanon, Saudi Arabia, Syria, Pakistan and Afghanistan, not to mention Israel, are all watching closely for missteps in Egypt, which could portend profound and lasting consequences for U.S. policy in the Mideast.

Fueled by a popular uprising that brought down the autocratic leader of Tunisia but has yet to cause wider alarm, the simmering anti-Mubarak sentiment in Egypt has boiled into a rebellion that might undermine the foundation of the Mideast's fragile stability, the 1979 Egyptian-Israeli peace accord.

But even without such a worst-case scenario, the ripple effects from a radicalized or even destabilized Egypt would be wide. From its control of the Suez Canal to its leading role in trying to calm tensions between rival Palestinian factions and rein in smuggling to Hamas militants in the Gaza Strip, Egypt has been a key U.S. strategic partner.

The collapse of such an ally would likely embolden U.S. foes around the world, including Islamic extremists in the Persian Gulf, North Africa, Central Asia, South Asia and Southeast Asia, many of whom are fighting secular authority.

As street protests and violence escalated in Cairo and other Egyptian cities on Sunday, Clinton took to the airwaves to appeal for Mubarak to finally heed decades of calls from successive U.S. administrations to open up political and economic space for his people. Dialogue and steps to ensure upcoming elections are credible are critical, she said.

But Clinton, as Obama has also done, stopped short of demanding the departure of Mubarak, a friendly but flawed leader whose government has benefited from tens of billions of dollars in U.S. largesse over the past three decades in spite of his refusal to date to adopt reform.

The U.S. wants to "real democracy" in Egypt, Clinton said, "not a democracy for six months or a year and then evolving into essentially a military dictatorship or a so-called democracy that then leads to what we saw in Iran."

Indeed, apart from the immediate concern of ensuring the safety of tens of thousands of Americans now caught in the Egyptian unrest, the 1979 Islamic Revolution in Iran coupled with more recent successes of militant Palestinian and Lebanese Islamist movements Hamas and Hezbollah weigh heavily on the minds of U.S. policymakers.

Egypt's Muslim Brotherhood, the country's most organized opposition group and one that does not share Mubarak's moderate, pro-U.S. agenda or approve of relations with Israel, may be best poised to fill a power vacuum.

Although Iran's was a Shiite revolt and Egypt is predominantly Sunni, there are similarities. Iran in the 1970s, like present-day Egypt, was a major U.S. ally whose support of American goals was criticized.

Clinton steered clear of mentioning that possibility but stressed that no "void" should be allowed to form.

"We do not want to see a change toward a regime that would actually continue to foment violence or chaos, either because it didn't exist or because it had a different view that it wished to impose on Egypt," she said.

"We also don't want to see some takeover that would lead not to democracy, but to oppression and the end of the aspirations of the Egyptian people," she said.

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EDITOR'S NOTE -- Matthew Lee covers U.S. foreign policy for The Associated Press.

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