After winning one of the toughest political fights of his career before Israel's parliament, Prime Minister Ariel Sharon faces a colossal battle to keep his government afloat, his party united and his plan to evacuate Jewish settlers and Israeli soldiers from the Gaza Strip on track, according to Israeli politicians and political analysts.

And now the uncertainty over the health of Palestinian leader Yasser Arafat, Sharon's longtime foe, has added another level of political uncertainty to the Israeli leader's ability and commitment to pursuing the evacuation plan.

"If Arafat lives, Sharon has to pull off the disengagement, but the potential for a Palestinian state in the West Bank decreases," said Hillel Frisch, a political scientist at the Begin-Sadat Center for Strategic Studies at Bar Ilan University outside Tel Aviv. "And if Arafat does not live, disengagement might be delayed, and there's going to be Palestinian infighting, which will give Sharon the excuse to say, 'I'm waiting.' "

Doctors in Paris, where Arafat was transported Friday, continued to investigate the cause of his deterioration, after initial tests found no signs of leukemia, said Leila Shahid, the Palestinian envoy to France, the Associated Press reported.

[Senior aides to Arafat said Sunday that tests had ruled out any life-threatening condition, the Reuters news agency reported.]

Sharon's spokesman, Raanan Gissen, said a change in the Palestinian leadership would allow Israel to "start the disengagement, and it's possible that what begins as a unilateral disengagement process may end up as a bilateral dialogue and negotiation over the future of this conflict."

While some analysts see Sharon's proposal as a way of avoiding participation in any peace process, they said that the emergence of a new, moderate Palestinian leader could force Sharon's involvement.

"If there's an orderly transfer of power, Sharon would be on the spot," said Yossi Alpher, co-founder of Bitterlemons.org, a Palestinian-Israeli Internet dialogue site. "There would be pressure on him at a minimum to negotiate the disengagement with this person."

Many Palestinians oppose Sharon's plan, which calls for closing all 21 Jewish settlements in the Gaza Strip, evacuating their 8,100 inhabitants and withdrawing Israeli troops, as well as vacating four settlements in the northern West Bank. The proposal was formulated without input from Palestinian leaders. One of Sharon's top advisers said its aim was to freeze any political process with the Palestinians and strengthen Israel's control over the West Bank.

Nevertheless, Sharon's disengagement plan represents the only significant proposal for change in the Israeli-Palestinian situation in more than a year. Israel has never withdrawn settlers from the Gaza Strip or the West Bank; its last pullout from any settlement was in 1982, when settlers were removed from the Sinai Peninsula under the terms of the Camp David peace accords with Egypt.

The Israeli parliament, the Knesset, approved Sharon's plan by a 67 to 45 vote last week after acrimonious debate, but the plan still faces legislative hurdles and cabinet decisions that could significantly alter or kill it before the withdrawals are to begin next year.

Four senior cabinet ministers from Sharon's hawkish Likud Party who backed the plan, including Finance Minister Binyamin Netanyahu, say they would quit the government if Sharon did not agree to put the plan to a nationwide referendum. If the National Religious Party, the smallest party in Sharon's coalition, were to follow through on a similar threat, his government would have 55 votes in the 120-member parliament, making it difficult for the government to survive.

"If Netanyahu leaves, if the National Religious Party leaves the government, we will have no government, no coalition," said Yuval Steinitz, a Likud member who is chairman of parliament's Foreign Affairs and Defense Committee and who voted with Sharon on Tuesday but supports a referendum. "Our ability to pass the budget is doubtful, and it's clear we're going to have to have new elections."

But in an interview published last week in Israel's largest daily newspaper, Yedioth Ahronoth, a defiant Sharon vowed: "There will be no referendum on disengagement, as the ones behind this initiative are interested in sabotaging the disengagement plan. I will never give in to pressure and threats."

Neither Sharon nor the four Likud ministers who threatened to resign -- Netanyahu, Education Minister Limor Livnat, Agriculture Minister Yisrael Katz and Health Minister Danny Naveh -- nor the National Religious Party, have left themselves much room to back down from their positions, according to politicians and political analysts. That sets the stage for more wrangling driven by politics, religion and ego, perhaps culminating in the collapse of Sharon's government and a call for early elections.

"Sharon doesn't like to be blackmailed," said Shmuel Sandler, a political scientist at Bar Ilan University. On the other hand, he said, "I don't see how Netanyahu can withdraw such an ultimatum." And if the National Religious Party pulls out, Sandler said, "Sharon has no choice but to go to elections."

The battle comes as the United States holds a presidential election, and many in the Arab world link the U.S. war in Iraq to President Bush's unbending support for Sharon in the Israeli-Palestinian conflict. Some of Sharon's opponents within Likud said Wednesday that his intractable opposition to holding a referendum stems in part from his determination not to disappoint the United States, his strongest international ally.

"He has to show stability vis-a-vis Bush," said Gila Gamliel, a Likud member of parliament who voted against the Gaza plan. "Bush is currently in a neck-and-neck race with [John] Kerry. He has to fulfill the demands of Bush."

Despite broad popular support, Sharon's Gaza plan was defeated in a Likud Party referendum in May and was voted down by the party's Central Committee in August. In an unprecedented slap, 15 of the 40 Likud members of parliament voted to repudiate a speech that Sharon gave this month on the opening day of the legislative session. On Tuesday, 17 members voted against the disengagement plan, forcing Sharon to reach out to traditional opponents in the parliament, or Knesset, to ensure the plan's approval.

Now, if the four cabinet ministers join the 17 party dissidents, Sharon will be left leading a minority faction of his party in the Knesset. His position would further deteriorate if others who support a referendum, such as Steinitz and Foreign Minister Silvan Shalom, joined the revolt.

But even if Sharon changed his mind after the U.S. elections, it is not clear that parliament would be willing to subject its own decision in favor of the withdrawal to a public referendum. Knesset vote-counters said a proposal to put the issue to referendum would not be approved.

"We oppose the referendum totally," said Giyora Ayalon, a spokesperson for the Labor Party's 19 Knesset members, who all voted for Sharon's plan. "All those who say that they support the referendum are in fact opposed to [the disengagement plan], and all they are interested in is gaining time and to try to obstruct the process."

Researcher Samuel Sockol contributed to this report.