TRIPOLI, Libya — The biggest danger to Moammar Gadhafi is not the rebel forces struggling to march on his capital. It's more likely to be the crumbling of the remaining, fragile support for his regime.

That is what makes the defection of Foreign Minister Moussa Koussa a heavy blow. He was part of a small circle of key insiders and family who have the most to lose if Gadhafi loses power. As those figures peel away, it makes the majority of his supporters, who have much looser ties, less certain that Gadhafi is capable of staying in power.

The Libyan leader relies most on his immediate family and his tribe, the Gadhadhfa. But his tribe is a relatively small one among the estimated 140 tribes that predominate life in the North African nation of about 6 million. So he vitally needs the support of others, whose allegiance he has bought over the years by handing their members top political and security posts.

Their loyalty is already fraying. International airstrikes hitting Gadhafi's forces — where these tribes make up much of the manpower — are designed in part to convince them that Gadhafi has to go.

Gadhafi's most important alliances have been with the Warfalla and Magarha tribes, thought to be among the biggest in the country, with some estimates of around 1 million members each. One of his right-hand men, military intelligence chief Abdullah Senoussi, is a Magarha (he's also Gadhafi's brother-in-law). Members of both tribes have filled the upper ranks of the security forces and government.

Warfalla and Magarha also largely fill out the militias led by Gadhafi's sons, Khamis, Muatassim and al-Saadi. The regime has relied on those forces to battle the rebels and besiege opposition-held cities because the Libyan leader feels assured of their loyalty. That means they have been main targets of the air campaign and are bearing the brunt of the punishment.

Some leaders in both tribes have announced their support for the anti-Gadhafi uprising since it erupted on Feb. 15, and numerous individual Warfalla and Magarha have joined the revolt, either as fighters or politicians. Mahmoud Jibril, the head of the opposition's eastern-based leadership council, is a Warfalla.

The rest in these tribes and others have remained with Gadhafi for the moment, whether out of fear of reprisals or because they hope to hold onto the perks and salaries the positions and military jobs provide them.

Recently, Warfalla figures within the regime — like infrastructure minister Maatouq Maatouq — have been sent to berate their fellow tribesmen to stay in line, said Faraj Najem, a Libyan historian and expert on tribes.

"They have made it clear to their tribe that anyone who speaks against the regime will be in trouble — that is, physically liquidated. So they have managed to suppress much Warfalla dissent," said Najem, who is based in London and is in touch with figures on the ground in Libya.

The allied tribes "can switch at any moment depending on the circumstances," Najem said. "There will be a moment when they will let Gadhafi down, either by turning against him or just stepping aside" and remaining neutral.

Since U.S. and European-led airstrikes on Libya began on March 19, Western officials have stepped up calls for Gadhafi loyalists to turn against him. "We call on all his supporters to drop him before it is too late," French President Nicolas Sarkozy and British Prime Minister David Cameron said in a joint statement Monday.

"Those who abandon Gadhafi in his crazy and murderous ways can join in the reconstruction of a new, democratic Libya," Sarkozy said.

The extent of sincere popular support for Gadhafi is hard to measure, given the overwhelming propaganda campaign his government has waged to show he remains beloved by the masses. Libyan state TV frequently shows "tribal gatherings" boasting of various tribes' backing for the man who has ruled for nearly 42 years.

Tripoli's Green Square is filled daily with partying loyalists honking their horns, waving green flags and toting posters of their leader. Music blares from loudspeakers and green flags flutter from the lampposts.

"Gadhafi brought us the wealth and freedom and happiness we are in," one man in the square shouted on a recent day.

In Gadhafi's private Tripoli compound, Bab al-Aziziya, dozens of Libyans that the government calls "voluntary human shields" mass each day, though their numbers have thinned out considerably since the allied airstrikes started. On Thursday night, they partied in the compound, waving flags and beating drums.

Many in the crowd were from Tripoli's Bu Sleim district, a pro-Gadhafi stronghold whose support for the leader has helped keep protests from spreading in the capital. "We are all here the sons of Bu Selim," shouted Mohammad Mansour over the loud pro-Gadhafi music. "We are all ready to defend the leader with our lives."

Najem, the historian, says there are three rings of support around Gadhafi. The closest are his sons and the senior insiders who have benefited most from his rule, who come from a variety of tribal backgrounds.

The second is the Gadhadhfa tribe itself, based in the central coastal city of Sirte with a significant presence as well in Sebha, a Gadhafi stronghold deep in Libya's southwestern desert. Over the past two decades, Gadhafi has increasingly planted Gadhadhfa members in key posts, particularly in the air forces.

These are the circles that will likely fight the longest for Gadhafi, Najem told The Associated Press, because they see their fates as tied to him personally.

The defection of Koussa, who was previously an intelligence chief, was just one sign that those power centers are crumbling. Weeks ago, the Libyan leader's cousin and close adviser, Ahmed Gadhaf al-Dam, abandoned the regime and fled to Egypt.

A major militia commander in the southeastern desert town of Kufra also defected, said an opposition spokesman, Mustafa Gheriani. Saleh al-Zewi commanded the Kufra contingent of the Khamis Brigades, named for the Gadhafi son who leads it, Gheriani said.

The Zweia tribe, to which al-Zewi belongs, has already announced its support for the uprising. It dominates a vast swath of eastern Libya, from the Mediterranean coast to the southeastern borders — where many of Libya's oil fields are located.

The weaker the inner circle seems, the safer Gadhafi's third circle of support, the allied tribes, may feel in breaking away definitively.

But intimidation — and uncertainty over who will win the conflict — can have a powerful hold on those tribes.

In Sirte, for example, the Gadhadhfa tribe dominates the city, but in fact it is outnumbered by another, rival tribe, the Firjan.

As rebel forces from the east marched on Sirte last weekend, opposition figures were in contact with the Firjan within the city urging them to rise up. Many Firjan already have, particularly their large populations in the opposition-held cities of Benghazi and Ajdabiya.

Among the opposition supporters trying to convince their brethren in Sirte to help them is Col. Khalifa Hafter, an influential Firjani who, as a commander in Gadhafi's forces, led the army in an offensive in neighboring Chad in the 1990s. Hafter is also the brother of the chief of the Firjan community in the rebel capital of Benghazi, Najem said.

Still, with the militia led by al-Saadi Gadhafi hunkered down in Sirte, the Firjan in the city did not rise up. A powerful offensive by Gadhafi troops drove the rebels back from Sirte and sent them retreating more than 100 miles over the past four days.

The attitude among many of the tribes that continue to at least nominally support Gadhafi appears to be to wait and see which direction the conflict turns.

That applies just as much to the Libyan leader's own Gadhadhfa brethren.

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Najem said he had been told by residents in Sebha, Gadhafi's southwestern stronghold, that some Gadhadhfa there have been buying up gold and making contingency plans to flee south across the border to Chad. The tribe is a minority in Sebha, and its prominence has fueled rivalries with local Magarha and members of another powerful tribe, the Hasawna.

"They know that without Gadhafi," Najem said, "the state won't give them cover any more."

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Keath reported from Cairo.