Now that euphoric hopes of an instant Middle East peace have evaporated, Arab officials are privately worrying about possible cataclysmic repercussions if, as if widely feared, President Anwar Sadat's go-it-alone diplomacy fails.

In Amman, Beirut, Damascus, Riyadh, and even Cairo, officials are coldly assessing the possible impact of a total breakdown in the Egyptian-Israeli negotiations or of a separate peace Sadat insists he rejects, but which he may wind up having to accept.

From political leaders as experienced as King Hussein of Jordan, Saudi Crown Prince Fahd and former Egyptian Foreign Minister Ismail Fahmi have come public warnings that either eventually could destabalize the notoriously vulnerable regimes of the Arab world.

The exact form of instability is hard to predict. But present in older memories is the string of revolutions, coups and general turbulence in the Arab world which followed the creation of the state of Israel in 1948.

King Hussein, a survivor of earlier splits in Arab ranks, was the first to warn of possible "convulsions and upheavals, a series of explosions whose cumulative impact could prove just as serious as another Arab-Israeli war."

Fahmi, who resigned to protect Sadat's initiative, feared an "escalation" of assassination attempts in the Arab world by Palestinian extremists who felt cheated by a separate peace.

Fahd, the effective power behind cautious Saudi Arabia's efforts to use its petrodollars to foster Arab unity, recently said "inter-Arab divisions are, in our opinion, more fatal than wars."

A common thread to such doomsday thinking is the possibility of a resurgence of Soviet influence in the region.

Already: Soviet influence, diminished by Sadat and former U.S. Secretary of State Henry Kissinger, is making a comeback. Diplomats report major Soviet arms shipments - included MIG-23s, improved Sam-6 ground-to-air missiles and T-62 tanks - on the way to Syria, paid for with Libyan cash, as part of a program to lend military crediability to the anti-Sadat front formed last month in Tripoli.

Turbulence in the front line states - Syria, Lebauon or Jordan - could well spuared to the Arabian peninsulia.

Traditionally, Syria is considered the "soft underbelly" of Saudi Arabia and the other conservative oil-rich states.

Damascus enjoys that role thanks to its central geography and reputation for pan-Arab radicalism, which the conservative oil states found embarrassing when earlier Syrian regimes attacked them as "reactionaries" and "feudalist."

Saudi Arabia's own reaction in case of a separate Egyptian-Israeli peace is unpredictable.

Cutting off the mammoth annual Saudi subsidy to Egypt - estimated at $4 billion - would present Sadat with a very real problem.

But the underpopulated oil-based Arab regimes in turn are well aware of their unhealthy reliance on Yemeni, Pakistani and especially Palestinian imported labor, which could be manipulated for subversive purposes.

Finally, Sadat himself may well set in motion regional destabilization by seeking to champion armed struggle if his peace efforts fails.

He justified his peace initiative by saying Israel was about to conduct a pre-emptive strike. Any major Egyptian change of heart about detente with Israel could turn that claim into a self-fulfilling prophecy.