President Anwar Sadat's rollercoaster diplomacy has lurched to a temporary halt after nine weeks with the world wondering increasingly if he will now make it to the end intact.

In a blaze of Egyptian-Israeli public invective his self-styled "sacred mission" has run out of steam. By his own admission, in a two-hour speech Saturday night, he appears to be relying on world public opinion to get Israel to agree to the minimum concessions he feels he needs to keep his peace initiative alive.

In practical terms that means help again from the United States, the superpower that was all but counted out in the euphoria surrounding his Jerusalem visit and predictions of the inevitable visit and predictions of the inevitable success of direct negotiations.

Yet, Egyptian foreign policy specialists reckon in private that the executive branch in Washignton has rarely been more disarmed in trying to promote a Middle East policy that meets Egypt's needs.

Watergate, Vietnam and other scandals having nothing to do directly with the Middle East have diminished the traditional clout of the White House, State Department and Pentagon. And Congress has taken up the slack to the Arabs detriment, Egyptian observers feel.

Signs abound in Egyptians eyes of the Carter administration's reluctance to be more forceful - including the President's reported remarks to Sadat on Wednesday - when the Egyptian peace talks delegation was recalled - that American ability to influence Israel was limited.

Sadat's appeal for U.S. arms in fact really disguised a more modest ambition since he understands that Israel's supporters would prevent any meaningful weapons deliveries. Rather, he is simply credited with wanting to prevent the added humiliation at a time of diplomatic deadlock of watching Israel contract for F-16 bombers.

(Israel has a request pending for 150 of the fighters, a figure reduced by 100 when the Carter administration rejected a plan to let the Israelis build some of the planes themselves to save money.)

Sadat doubtless remembers how short-lived such a freeze might be. In 1975, former Secretary of State Henry Kissinger withheld promised arms deliveries from Israel in punishment for Jerusalem's rejection of an early version of the Sinai II disengagement deal. Once the deal was signed, however, the promised arms - plus some - began flowing.

Nevertheless, Sadat still hopes that the United States will act - not just out of compassion to save him and his moderate government, but to preserve its vital national interests in the Middle East as American oil imports from the region continue to grow.

In part, Sadat finds himself depending on American support because his diplomacy has failed either to disarm his radical Arab critics or convince the moderates to rally to his patently eroding cause. Perhaps the Egyptians may be forgiven for wondering why the United States cannot take a more forceful stand against the Israeli settlements on occupied Arab land which successive U.S. administrations have denounced as "illegal."

The spectacle of American inaction in Arab eyes prompts some Eyptians to wonder if and when Sadat may turn against the United States.

Deterring Sadat from any such course is his consistent policy over the past six years of burning his bridges with the Soviet Union, thus depriving him of a source of new arms and spare parts for his Soviet-equipped army and an alternative to his dependence on the United States.

With some Egyptian convinced that his position is eroding so fast they talk of his "political countdown," the best the government seems able to muster is private talk of hoping the Americans "will get the pipeline together since we have to continue the talks."

Indeed they may continue, but perhaps Egyptians should be forgiven in their present state of letdown for assuming skeptically that future talks will be more restricted and less meaningful than they had originally hoped.

The really cynical claim they have abandoned hope, in one man's words, "until after the Begin period in Israel and the Carter administration in America."

Left unsaid is the probability that Sadat would not be around, in their view, to see the triumph of the ideas he set in motion.