With the winter session of parliament just three days old, Prime Minster Ariel Sharon is headed toward a showdown over his plan to withdraw Israeli troops and Jewish settlers from the Gaza Strip, and some of his toughest opponents are members of his own party.

Many members of the Likud Party, which soundly defeated the proposal in a nationwide party referendum in May and again at a party convention in August, were furious when Sharon announced in a speech at Monday's opening session of the Knesset, Israel's parliament, that he would bring what he calls the Gaza disengagement plan to a vote in the legislature on Oct. 25. In a symbolic rebuke, lawmakers voted 53 to 44 to reject the speech, with 15 of Likud's 40 members in the majority group.

Political analysts said the three drubbings caught Sharon -- widely considered a masterful tactician and brilliant politician -- flatfooted and unprepared to confront the political forces lined up against him, particularly powerful Jewish settler groups that oppose his plan to remove all 8,100 settlers from Gaza's 21 settlements. While few people predict his ultimate defeat, many say that Sharon's continued pursuit of the disengagement plan risks a split in Likud, the collapse of his government or the death of the plan -- even though both Sharon and the Gaza withdrawal have widespread support among Israeli citizens.

"Why was Sharon so smart politically and managed so well when he had only 19 members in the Knesset, and now when he has 40 seats he stumbles and runs from one failure to another?" said Nahum Barnea, an Israeli political analyst and newspaper columnist. "He failed because he didn't appreciate the extent to which the [Likud] rebels are committed to other groups outside the Likud," particularly Jewish settler organizations.

Angry at the defections, Sharon responded Tuesday by renewing negotiations to bring new partners, such as Labor, the main opposition party, into his fragile three-party coalition government -- a move that was explicitly prohibited by Likud's Central Committee at its August convention. In key convention votes, Likud members said Sharon could not negotiate with Labor leaders about joining the government precisely because Labor supported the Gaza withdrawal.

Sharon warned the convention that Likud was on "the verge of division and disintegration," adding: "We have to decide whether the Likud will continue to lead the state united with responsibility, or the Likud will be led by an extreme, irresponsible, rebellious opposition."

Little has changed since then. Sharon's coalition, which has survived at least 56 no-confidence motions since he proposed the Gaza withdrawal last December, lost its parliamentary majority in June. He tried for months to broaden the coalition, but other parties are so divided over the disengagement plan, budget priorities and religious differences that they refuse to sit under the same umbrella.

"He has a minority government to govern. He has a majority in the Knesset to pass the budget and a different majority to pass the disengagement plan," said Hanan Crystal, a political analyst. "But this situation, with two coalitions -- one to govern with the rebels and another to pass the disengagement plan in parliament -- cannot last for long. Something will kill it."

Vote-counters say that even if the 15 Likud rebels abandon Sharon in the critical vote on disengagement, the plan will still have enough support to pass because of the presumed backing of Labor and other moderate parties. But in that case, Sharon's coalition could collapse because one member, the National Religious Party, has vowed to pull out if the plan is approved.

Sharon currently is trying to fashion a new government with Labor, which has 19 members in parliament, and Shas, an ultra-religious party with 11 seats, which would give him a comfortable majority of 70. But Shas opposes the disengagement plan and is refusing to join, and Labor does not like Sharon's market-oriented economic policies and could demand key cabinet posts -- such as the finance and foreign ministries -- that now belong to Likud stalwarts Binyamin Netanyahu and Silvan Shalom.

Members of Labor and other moderate parties were angered by an interview published last week in the daily Haaretz newspaper in which Sharon's former chief of staff, Dov Weisglass, said the disengagement plan was designed to freeze the political process between Israel and the Palestinians and strengthen Israel's grip on Jewish settlements in the West Bank.

Netanyahu and settler groups are pushing for a nationwide referendum on the disengagement plan, and recent public opinion surveys indicated that about two-thirds of all voters support it. But Sharon opposes a referendum, seeing it as a thinly disguised strategy to stall it. Israel does not have a law regarding public referendums, and drafting and approving such a bill could take more than a year, Sharon's supporters say.

Others see the idea as a natural compromise.

The Likud cabinet minister, Natan Sharansky, said that if Sharon follows his current course, hundreds of thousands of protesters could flock to Gaza to try to block any withdrawal.

"But if there's a referendum, and all the people speak and the feeling is that it's the will of the majority and it's not being imposed upon me by a prime minister, then maybe instead of 200,000 demonstrators you'd have only 5,000, because we are a strong democracy," Sharansky said.

Prime Minister Ariel Sharon has angered some in Likud.