In the heady public posturing that usually masks the true nature of their politics, it has always been a cardinal article of public faith that Yasser Arafat's Palestine Liberation Organization has no greater champion, or friend, than Syrian President Hafez Assad.

When the cameras are turned on the two leaders at periodic Arab summit conferences or their own regular one-on-one meetings in the Syrian capital of Damascus, they invariably are shown embracing warmly and chatting amiably. Arafat, dressed in his traditional checkered kaffiyeh, and Assad, in his French-cut business suits, incessantly speak to their public of the brotherhood between the Syrian and Palestinian people.

But it is no secret that the relationship between Arafat, the guerrilla chief, and Assad, his hard-nosed benefactor, has never been an easy one. In the age-old ways of the desert that spawned their culture, it is suspicion, not trust, that rules their political relationship.

"It has always been a marriage of convenience," one PLO official in Beirut said, asking that his name not be used. "We Palestinians detest the Syrians and they detest us. But we have to live together, all the same."

Life together, of late, has become more trying than normal. PLO officials are quietly blaming Syria for orchestrating an underhanded campaign to discredit Arafat at the very time he is trying desperately to present himself to the world as the diplomatic statesman he would like to be, rather than the gun-wielding terrorist his enemies in Israel insist he will remain forever.

PLO sources say the campaign against Arafat includes plots against PLO diplomatic representatives in Europe, probably even the shooting in Warsaw of Arafat's PLO colleague Abu Daoud, and an embarrassing attempt to tar the PLO with a terrorist attack against a synagogue last month in Vienna. In some knowledgeable circles here, it is even alleged that the anti-PLO campaign led to the Sept. 4 assassination of the French ambassador to Lebanon, Louis Delamare, whose only crime may have been his escort just days before his death of French Foreign Minister Claude Cheysson to a meeting with Arafat in Beirut.

PLO suspicion that Syria is behind all these events hinges on the hospitality Syria has extended recently to a disparate collection of anti-Arafat Palestinian dissidents grouped around a Palestinian renegade named Sabri Banaa, who goes by the code name of Abu Nidal.

In the Arab world, Arafat probably has no more implacable enemy than Abu Nidal, a former member of his own Fatah organization whom Arafat expelled in 1972 and later sentenced to death in absentia for his violent opposition to any consideration of a negotiated settlement of the Arab-Israeli conflict. Abu Nidal's heresy was to insist that the gun and not the olive branch was the only possible solution to the Palestine question.

Even in a region where political alliances shift with the ease of sand dunes in a desert storm, Abu Nidal stands in a class by himself.

Outlawed by the PLO, the umbrella organization of Palestine resistance movements that Arafat has chaired for 12 years, Abu Nidal and a hardened band of followers soon turned up in Iraq. They called their organization the Fatah-Revolutionary Command and, according to Arab and Western intelligence sources, were soon working for and dependent on the Iraqis.

From 1976 to 1978 Abu Nidal's group was held responsible for a series of terrorist acts in Syria, including a 1978 assassination attempt on Syrian Foreign Minister Abdul Halim Khaddam. The ruling Baath parties in Syria and Iraq are fierce rivals.

During the same period Abu Nidal was linked to assassinations of PLO diplomatic representatives in London, Kuwait, Paris and the Pakistani capital of Islamabad.

Having been condemned to death in absentia by a PLO tribunal for these acts, Abu Nidal replied by issuing his own death verdicts against Arafat and his second in command, Salah Khalaf, usually known as Abu Iyad.

Following Egyptian President Anwar Sadat's peacemaking trip to Jerusalem in 1978, Arabs opposed to Sadat's action tried to put aside their internal disputes. Baghdad, in a gesture of reconciliation with Arafat, agreed to rein in Abu Nidal and curtailed his activities. He was quietly expelled him from Iraq sometime in 1979.

Unheard of for several years, Abu Nidal's group, reinforced with other dissident Palestinians, made one of the great political turnabouts in the Arab world, suddenly cropping up in Syria this year, according to both PLO and Western diplomatic sources. As independents are given no more leeway of action in Assad's Syria than they had in President Saddam Hussein's Iraq, it has been assumed by PLO leaders that Abu Nidal, despite his earlier war on Assad's regime, has been given Syrian intelligence protection of some sort to carry out missions that Syria approves.

The first signs that he was again going after Arafat and the PLO came last spring when the PLO uncovered a plot against their representative in the Yugoslav capital of Belgrade. Following that discovery four Palestinians, including one who had infiltrated the Belgrade PLO office, were arrested. The suspects, according to the PLO, were found to have links to Abu Nidal.

In June, Abu Nidal was blamed for the assassination of Naim Khader, the head of the PLO in Brussels. When Abu Daoud, the PLO guerrilla leader held responsible for the planning of the 1972 massacre of Israeli athletes in the Munich Olympics, was shot five times in a hotel in Warsaw Aug. 1, PLO investigators suspected Abu Nidal rather than the Israelis that have so long been hunting him.

Arafat's greatest embarrassment, however, has come from actions alleged to be sponsored by Abu Nidal in the Austrian capital of Vienna. The choice of Austria, the PLO sources say, could not have been more calculated. For Prime Minister Bruno Kreisky, who in 1979 became the first Western European leader to give the PLO official recognition, has been a key figure in encouraging Arafat to seek a negotiated peace with Israel.

Abu Nidal's group has been blamed with the murder this summer of the head of the Austrian-Israeli friendship society in Vienna, an effort to infiltrate two well armed guerrillas into the city on the eve of a state visit by Egyptian President Sadat (which was canceled for security reasons) and the Aug. 29 gun and grenade attack by two Arabs on a Vienna synagogue that left two dead and 20 injured.

The synagogue attackers, who made a point of identifying themselves publicly as members of the PLO despite Arafat's public disavowal of the raid, have since been identified by Austrian police investigators as members of Asifa, the military arm of Abu Nidal's movement. Last week, Agence France-Presse reported from Paris that Asifa had claimed responsibility for the Vienna attack.

Prime Minister Kreisky, who accepted Arafat's disavowal of responsibility, said last week that the attack had been conducted by "a small group of Palestinians, in the service of one or two Arab states."

PLO officials privately make no bones about the fact that at least one of the states in question is believed to be Syria, which they say is trying to discredit Arafat because of his persistence in such independent initiatives as his commitment, without prior Syrian approval, to a ceasefire with Israel in southern Lebanon on July 24.

In fact, it is Syrian President Assad's well known displeasure with Arafat's recent independent diplomacy that has led many knowledgeable Arab sources--and at least one Beirut newspaper--to blame Syria for the assassination, after the apparently foiled kidnap attempt of the French ambassador.

The official investigation into the killing, which took place within sight of the ambassador's official Beirut residence and Syrian peacekeeping troops who were maintaining roadblocks--one of which the assassins drove through unhindered after the shooting--is still underway. But given the political delicacies of Lebanon, its conclusions may never be published.

And though in this land of clandestine politics and cross-accusations, it is even possible that the PLO has put out the story for its own purposes, it is being alleged privately around Beirut that Delamare's killing might be linked to the fact that Cheysson met Arafat in Beirut, and not in Damascus, where Cheysson went after he left Lebanon. This fact, and the apparent laxity of the Syrian roadblocks after the killing, is being cited by sources, including the newspaper Le Reveil, to justify suspicions that a plan to kidnap Delamare could have had at least tacit acceptance of Syrian intelligence groups who wanted only to embarrass Arafat and shake up the French.

Delamare's actual death is considered to have been a gross mistake resulting from panic rather than conspiracy.

The immediate roots of the current dispute between Arafat and Assad go back at least a year, when Syria, in support of Iran's war with Syria's own antagonists in Iraq, forced the PLO, and other members of the so-called Arab rejectionist front, to boycott an Arab summit meeting in Amman, Jordan, that had been called specifically to deal with the PLO's raison d'etre, the Israeli issue.

Since that time, Assad and Arafat have been bitterly at odds in private about their relations, while professing to be brothers in public.

PLO sources complain that Assad is not willing to allow Arafat any independence of action that would undermine his own cherished role of primacy over the PLO, which depends on Syria for arms and security in base areas in Lebanon where 30,000 Syrian troops are stationed as members of the Arab League peacekeeping forces.

While no one has yet linked Abu Nidal with the French ambassador's death, PLO sources privately are saying that Nidal's actions abroad are not necessarily distinct from policies conducted by other Syrian proxies in Lebanon. The use of proxy forces in Middle East politics, especially in anarchic Lebanon, is a popular tool, because such forces can be publicly disavowed by their masters or, if necessary, even eliminated.

What enrages PLO officials in Beirut about Abu Nidal's actions is the belief that they can only be conducted with Syrian approval.

"In Damascus nobody operates on his own," a PLO official maintained. "They only operate when it is in the interest of their government to do so."