Earlier this month, Yemen’s president Ali Abdullah Saleh promised demonstrators that he would not seek another term in his three-decade rule, committing to leave power by 2013 and hold open elections.   Saleh’s pledge has not appeased protesters, and now his own power base appears to be falling apart.  Two chiefs from his own tribe have publicly changed sides and now demand that Saleh leave office immediately:

Two powerful tribal chiefs in Yemen joined opposition forces demanding the ouster of the country’s longtime president Saturday, a new sign the embattled leader might be losing his grip on the impoverished, conflict-ridden country.

The defections were a blow to President Ali Abdullah Saleh because the chiefs abandoning him are from his own tribe, the Hashid. That was a signal that he may not be able to capitalize on tribal rivalries to maintain power, a tactic he has successfully employed in the past.

They’re not leaving quietly, either:

In a speech to the protesters, Sheikh Hussein bin Abdullah al-Ahmar, a key Hashid leader and a longtime ally to Saleh, said he is resigning from the leadership of the ruling party.

“I call on every honorable Yemeni to work to topple the regime” said al-Ahmar to the applause of the protesters, many carrying weapons. “The regime should go and be replaced by state institutions.”

This could be the Obama administration’s biggest headache in the Arab uprisings of 2011, assuming they don’t spread to Saudi Arabia.  In Tunisia, the uprising came from a fairly Westernized populace seeking an end to a kleptocrat.  In Egypt, the Muslim Brotherhood may well play kingmaker in democratic elections, assuming the military will actually relinquish power, but other democrats can and will likely organize along secular lines and limit the Ikhwan’s influence.  Libya has melted down into open civil war against a terror-sponsoring regime hostile to the US, but despite Moammar Gaddafi’s hyperbolic claims, an al-Qaeda coup is almost entirely unlikely.

That’s most certainly not the case in Yemen.  AQ has been actively undermining Saleh for years, and its Yemeni branch has grown more active and dangerous over the last few years under the leadership of American citizen Anwar al-Awlaki.  Saleh’s government has reluctantly and inconsistently aligned itself with the US but has attempted numerous rapprochements with AQ, with several prisoner “escapes” that seem highly suspicious.  Saleh also publicly refused to extradite Awlaki even if the Yemeni security forces managed to capture him.  Appeasement hasn’t worked, and Saleh allowed the US to conduct military raids on AQ targets over the past couple of years to slow down their growth, but have mainly missed the leadership of the terrorist network, as several plots against the US have proven.

The sudden instability of Saleh complicates another priority for Barack Obama as well.  He came to office promising to close the detention center at Guantanamo Bay, which now holds almost 100 Yemenis captured by the military or intelligence agents.  Yemen already has a poor record in keeping released Gitmo detainees under wraps, and they have a flood of AQ recruits coming into the country without the US sending more.  It’s almost certain that whatever replaces Saleh’s regime won’t continue even the limited policies of cooperation with the US, which means the White House really has nowhere to send the Yemenis at Gitmo.

When Saleh falls, the US will face a nightmare scenario of AQ opportunity to seize control of a state, and one in a critical location on major sea lanes and near the richest sources of energy in the world.  Their first target will be Saudi Arabia and an attempt to seize Mecca and Medina by overthrowing the oppressive monarchy in Riyadh, Osama bin Laden’s ambition since starting AQ two decades ago.  Will Obama be prepared to deal with this outcome?  Given his disturbingly passive response to the uprisings thus far, confidence won’t exactly be high.