London — Saudi Arabia's emerging diplomatic stance has suffered an undoubted setback: At the Arab summit in Fez in late November the kingdom was publicly revealed as unable to ''deliver'' consensus Arab backing for Crown Prince Fahd's Mideast peace plan.
Now, a somber Arab world is reviewing once again the development that led to the Fez fiasco. Their somberness derives from a new realization both of the constraints on meaningful Arab diplomacy, and of the awesome responsibilities now facing the hard-line Arab bloc that stymied the Saudis.
In particular, the Syrians and their allies in the Steadfastness and Confrontation Front fear that the appearance of Arab disarray might tempt the Israelis to launch a new military adventure against Arab targets.
In Saudi Arabia, meanwhile, there is reported to be a growing lobby inside the ruling family for shelving the whole idea - not just the Fahd plan but also the possibility of the kingdom ever putting itself in such an ''up-front'' diplomatic position again.
Saudi diplomacy, proponents of this view argue, has always been most effective when least exposed to the public gaze. A trusted Saudi prince may have a discreet word with a minister here, dispense a check or two over there, and things would slowly take their expected course. . . .
Even if this is too rosy a view of the traditional reality, many Saudis would still be happier with such a backstage role than with the humiliating loss of face the kingdom has now had to suffer.
The kingdom's policymakers are nevertheless looking at the crucial areas where the Fahd initiative failed. Why did the negotiations with the Syrians, which continued virtually till the eve of the summit, come to nought? Why did the Iraqis, who had been shifting toward Saudi positions in many fields over recent years, finally come out against them? Why was Yasser Arafat unable to ''deliver'' the Palestine Liberation Organization backing he appeared to have promised?
And, crucially for the Saudis, why did their own friends in the Gulf Cooperation Council fail to give concrete sponsorship to the plan?
In each case, the mix of reasons was different. But a number of themes ran through the reasoning of most of these leaders:
* The whole way the Saudis presented the play was wrong, they say. Ever since President Anwar Sadat dramatically broke Arab ranks to visit Jerusalem in 1977, the other Arabs have been fearful of anyone else peeling away from the consensus to join him. At their 1978 summit in Baghdad, Iraq, they decided that no Arab peace initiative should be made public before full consultation.
* Given that the Saudis had contravened this agreement, the Syrians and others said, they might perhaps still consider the plan if it had good chances of US backing. The Saudis reportedly went out of their way to reassure President Hafez Assad's envoy, his brother Rifaat, that the plan had not been produced jointly with the Americans. ''We would prefer it if it had,'' Rifaat is said to have responded.
* The United States, meanwhile, was not giving out any signals the Arabs considered to indicate a serious desire to push forward a peaceful Mideast settlement. They noted that the official US view considered only its clause allowing for the future recognition of Israel as positive - the very clause they themselves found hardest to stomach. The US, meanwhile, made no comment on crucial issues such as Israeli withdrawal from the occupied territories also mentioned in the plan.
Just a few days before the Fez meeting many Arab statesmen were bitter that President Reagan expressed a preference for Jerusalem to remain the unified capital of Israel. And they saw the US moving right into a ''strategic alliance'' with Israel.
* Saudi oil policy has meanwhile been hurting many of its fellow Arabs. Revenues have been falling not only for the radical governments in Algeria and Libya, but also in the traditionally pro-Western oil states of the Gulf. The Iraqis, too, desperate to disentangle themselves from the war with Iran, feel that world efforts for a cease-fire along their oil-rich battle zones would have been far greater if the Saudis were not ''flooding the market with cheap oil.''
* The shadow of President Sadat's killing hung over Arab calculations, too. All Arab rulers know that the fundamentalists who cut the Egyptian leader down are growing in influence throughout the Mideast and North Africa. Mr. Sadat was killed for making too many concessions to Israel for the fundamentalists' liking. Other rulers, too, could be at risk by making too many concessions with nothing concrete in return.
(This feeling is thought to have been a potent factor inside Saudi Arabia, adding to the reluctance of many Saud family members to give outright backing to the plan. A large part of the Sauds' claim to legitimacy as rulers is their defense of Islam. And since their own Muslim hard-liners occupied the Grand Mosque in Mecca in 1979, they have been treading a tightrope between fundamentalism and the demands of modernization.)
* Behind all these points the radicals were starting to look ahead to April. Then, after the scheduled Israeli withdrawal from Sinai, there will be more possibility of Egypt returning to Arab ranks.
But would Egypt return to a Saudi-dominated Arab fold? The prospect of facing such a strengthened conservative Arab bloc horrified the radicals, who still hope Egypt's President may tilt further their way. They wanted to be sure any Arab bloc welcoming Egypt would be firmly in the radical camp.
The conjunction of all these factors - rather than much in the actual text of the Fahd plan - made the Saudis' diplomatic rout at Fez relatively speedy. And now the radicals are looking ahead to press home the advantage they have gained.
They will be soberly aware of their constraints in doing so. There is still ill feeling between Iraq and the five members of the Steadfastness and Confrontation Front. None of the radicals wants to force a total break with the Saudis.
But the greatest handicap for the Arab radicals is their own failure to formulate a viable alternative. No Arab state except Egypt ever considered Camp David valuable. And until Jordan reveals more about a rumored peace plan, the failed Fahd plan remains ''the only ball game in town.''
Most Arab regimes still express deep interest in reaching a peaceful regional settlement. But the snowballing influence of fundamentalist Islam has already limited their room for diplomatic maneuver.