Following the assassination of Egyptian president Anwar Sadat, militant Jewish settlers in the occupied northern Sinai Peninsula have vowed to accelerate their campaign to force the Israeli government to abrogate the Egyptian-Israeli peace agreement and cancel the scheduled April 25 withdrawal from the Sinai.

The settlers warned that the political instability of Egypt, as evidenced by the assassination and the rise of violent Islamic fundamentalism, makes it imperative for Israel to stop the withdrawal and retain at least part of the strategic depth of the Sinai.

"I firmly believe there will be no evacuation of the area next April. The situation is too unstable," said Uri Elitzur, secretary of the Stop the Withdrawal from Sinai movement, which says it has collected 300,000 signatures to a petition favoring abrogation of the agreement. The group says its goal is a million names, nearly a third of Israel's population.

Ella Weizman, a spokeswoman for the movement in Sadot, in northern Sinai, said, "We've always asked what if something happens to Sadat. How can Israel rely on the treaty if it doesn't know who is going to be in charge in Egypt from one day to the next?"

While the Stop the Withdrawal movement respresents, so far, a minority within Israel, Sadat's assassination has presented Prime Minister Menachem Begin with new complications in his plans for orderly compliance with the terms of the peace agreement.

Even before Sadat's death, the narrow spit of inhospitable desert in the northeastern corner of the Sinai Peninsula -- called the Rafiah Salient -- had acquired the ominous sobriquet of "Begin's Altalena."

The reference is to the gun-running freighter that Begin's terrorist underground unit, the Irgun, used to challenge the authority of the newly installed government of Israel on June 20, 1948, in an attempt to scuttle a tenuous truce that had been made with the Arabs. The Altalena was sunk on orders from David Ben-Gurion in the first infamous battle of Jews against Jews in the new state, and Begin has never forgotten the bitterness of the moment.

Now that the Egyptian-Israeli peace that was so carefully nurtured by Sadat is being put to the test by the uncertainties arising from the assassination, the Rafiah Salient and its 4,000 Jewish settlers could become as important to the future of peace in the Middle East as was the Altalena to the future of the newborn state of Israel three decades ago.

On April 25, according to the Egyptian-Israeli peace agreement resulting from the Camp David talks of 1978, Israel must withdraw its armed forces and civilian settlers from the last third of the Sinai, an irregularly shaped swath of desert stretching from El Arish on the Mediterranean coast to Ras Mohammed on the Red Sea.

Contention has focused on the small, strategically important Rafiah Salient just below the Gaza Strip, and it is there that Begin faces the most crucial political test of his four years in office. Even before Egypt's future became uncertain, ultranationalist settlers had vowed to dig in and, if necessary, physically resist a forced evacuation from their homes, even if it means scuttling the peace agreement.

In their pronouncements since Sadat's death, Israeli officials have taken pains to reassure Egypt and the world that all of the remaining occupied Sinai, including the Rafiah Salient, will be evacuated by Israel on schedule, providing that the peace process Sadat started continues.

For his part, Begin has alluded to the issue only obliquely, saying he "hopes" the peace process will continue and, by implication, pledging an Israeli withdrawal.

But pulling back from the Sinai may turn out to be easier said than done.

First, there are the promises of the ultranationalist Gush Emunim (Bloc of Faithful) settlement movement, which has a degree of influence on the pivotal National Religious Party in Begin's fragile parliamentary coalition, to send thousands of settlers to the Rafiah Salient to block an evacuation. Already, scores of militant Jewish settlers from the West Bank have squatted in abandoned houses in the Sinai development town of Yamit and in the Talmei Yosef settlement for a showdown with any Israeli security forces that attempt to remove them, and some extremist settlers say they have stockpiled weapons with which to fight.

Some members of the National Religious Party's conservative faction found in the party's campaign platform a clause declaring that if the Egyptian-Israeli situation changes, the government should reconsider leaving the Sinai. Based on that, National Religious Party ultranationalists are demanding a meeting of the party's central committee to reconsider the peace agreement.

The question being asked by many Israelis these days is whether Begin, given his proximity in viewpoint to the settlers on so many territorial and historic issues, could bring himself to order the Army to confront the holdouts physically, facing the possibility of bloodshed.

Israeli border police and Army regulars have been used in the past to remove Jewish settlers forcibly from illegal encampments in the West Bank, but those confrontations never reached the level of violence feared in a forced Sinai evacuation.

Secondly, there has been growing anxiety in Israel's defense establishment that the U.S.-sponsored construction of two new Israeli air bases in the Negev desert, designed to replace two Sinai bases that are to be evacuated, may be falling behind schedule. The construction delays mean that Israel could be faced with the dilemma of either postponing the Sinai withdrawal or putting up with a temporary loss in Air Force combat readiness.

Thirdly, Begin faces a growing movement within his Likud bloc in the Knesset (parliament) that believes that the sacrifices Israel made and the gamble it took by agreeing to withdraw from the Sinai have been magnified considerably by Sadat's death, and that the evacuation should at least be reconsidered by parliament.

Originally led by the ultraconservative Tehyia Party, whose three Knesset deputies openly advocate an abrogation of the Camp David agreements, the movement has spread to Begin's own political nucleus in Likud, the Herut Party. One of its members, Moshe Arens, chairman of the Knesset's foreign affairs committee, said he hoped the new Egyptian president would support peace, but added that "we will have to stand guard, study the new man, increase our vigilance and evaluate the whole situation anew."

If the withdrawl issue comes to a debate in the Knesset, it could be as acrimonious and divisive as the 1979 debate over ratification of the Camp David agreement. Begin, painfully aware of his razor-thin parliamentary majority, is not likely to relish that prospect.