President Carter's once-forceful Mideast peace plan has deteriorated so rapidly that Israel's more thoughtful American backers fear what they call Prime Minister Menachem Begin's "creeping annexation" of the West Bank will go forward, without effective interference from Washington, to a climax of blood and tragedy.

This view may be premature and unfair to Carter. But it is held by the shrewdest American Jewish leaders, many of whom have deep, unpublicized reservations about Begin's zealous commitment to a "greater Israel."

One prominent Jewish leader told us that Begin's force of personality and blunt refusal to accept the Carter peace plan of last spring "has caused Carter to retreat to point zero." It is a view widely held in Washington.

The latest ingredient giving substance to this unflattering view was the President's own inexplicable phraseology in an exclusive interview he gave to Trude B. Feldman, widely known writer on American-Jewish affairs, published in the Jerusalem Post Sept. 9.

After detailing the "enormous" time he said he had spent on the Middle East, Carter dropped a bombshell: "If I see no further interest among the parties in a settlement, and if I become convinced they're not acting in good faith, I will be reluctant to continue my efforts to bring them together."

That dark hint of U.S. pullback from active participation in Mideast negotiations sets the stage for "creeping annexation" of the West Bank by hardline Israeli expansionists who now hold power. Even Israel's friends here acknowledge its overwhelming military superiority over any conceivable combination of Arab states today. Thus, if U.S. pressure on Israel - boldly exerted by Carter during the first months of his presidency - were suddenly removed, there would be no visible resistance to Begin's expansionist plan.

Begin's hand is strengthened, therefore, by the President's hint that if Israel refuses to make U.S. desired concessions he will pick up his marbles and move out of the Mideast. Ignoring Carter's fervent appeal, Begin had legalized three settlements and authorized three more since his visit here last July. The Carter administration, except for repeating old formulations stressing its opposition, has done nothing to inhibit Begin - not even hinting at reducing multi-billion dollar U.S. aid for Israel.

Those thoughtful leaders of the American-Jewish community we have been quoting fear this inaction from Washington. While admiring Begin's toughness, they know his politics too well to accept the thesis that planting settlements on the West Bank is simply a lever to push the Arabs into overall peace concessions.

To the contrary, Begin's policy is correctly perceived by these Jewish leaders as creeping annexation under the law of adverse possession: The longer the Israelis stay in the settlements and the more Israelis settle there, the stronger their claim to perpetual ownership. Down that road lies the certainty of war.

Fearful of what a new war might mean for U.S. Israeli relations and for Israel itself, these American Jews are starting to blame the President as much as Begin for the dangerous deterioration of the Carter peace policy. The seeds of this concern were planted July 6, when Carter, bidding for jewish support just before Begin's visit to the White House, invited a score of Jewish leaders to the White House for a long meeting.

He astonished them by appearing to give in to every request they made. "When we go to see a President we naturally demand the moon," one Jewish leader told us, "but we know we won't get it. With Carter, we got it."

Inept at playing subtle domestic politics with the American-Jewish community, Carter has upset the precarious balance other Presidents have maintained. The world's misfortune is that he has done so coincident with the advent of the most expansionist prime minister in the history of modern Israel.

While an increasingly large minority of Jewish leaders are concerned that Carter is giving Begin far too much rope, no public rebuke of the President from the American-Jewish community is remotely possible. Whoever expressed such sentiments would be swiftly denounced. Instead, the stage is being set for an enveloping tragedy in the Middle East with implications for the United States, Western Europe and Japan of devastating proportions.