Prime Minister Ariel Sharon (searchleft for Washington on Tuesday to win U.S. backing for a plan to give up the Gaza Strip (searchand parts of the West Bank (searchin exchange for keeping and expanding five large West Bank settlement blocs.

Sharon disclosed the final element of the plan just hours before his departure. In a visit to the West Bank's largest settlement, Maaleh Adumim (search), he listed for the first time the West Bank areas he wants to keep. About 92,500 of the approximately 220,000 West Bank Jewish settlers live in the five blocs listed.

President Bush apparently has no problem with Israeli withdrawals — on Monday he said a Gaza pullout would be a "positive development" — but it appears unlikely he is ready to declare now that Israel can keep parts of the West Bank. Such a declaration would violate the U.S.-backed "road map" peace plan, which says the borders of a Palestinian state must be negotiated by both sides.

Bush reiterated Monday that "if Israel makes the decision to withdraw, it doesn't replace the road map."

Still, Sharon is hoping Bush can help him persuade hardliners in his Likud Party to back a withdrawal. Some 200,000 Likud members are to vote on the "disengagement" plan on April 29, and approval is not assured.

If Sharon fails, he could come under pressure to resign.

Sharon has said he would honor the outcome of the vote, but has not spoken about resigning if he loses.

However, his vice premier, Ehud Olmert, on Tuesday referred to such a possibility. If the Likud members vote no, "they are destroying the political basis of the government headed by Sharon," Olmert told Israel Army Radio. Olmert said some opponents of the disengagement are trying to topple Sharon.

According to senior Israeli government officials, who spoke on condition of anonymity, Sharon and Bush will exchange letters at their Wednesday meeting, outlining commitments to the withdrawal plan and to the road map.

Sharon's letter will say that Israel will pull out of all 21 Gaza Strip settlements and four isolated West Bank enclaves, the officials said. In exchange, Bush will say Israel has a right to "pursue terrorists," including in areas from which it has pulled out, the officials said.

Israeli officials have said the United States has turned down Israel's request for recognition of West Bank settlement blocs. Olmert said Tuesday that he doesn't expect formal U.S. backing of Israel retaining the five settlement blocs Sharon listed Monday night, but that he is confident Washington will give a general OK.

"I wouldn't be surprised if this will be expressed, in one way or another, in the joint statement to be made by the president and the prime minister," Olmert said.

On another Israeli demand — backing for Israel's position that Palestinian refugees not be allowed the right to return to their former homes in Israel — Sharon could at best expect a murky statement, Israeli officials said.

"There appears to be nothing new," Yossi Alpher, an expert on Israel-U.S. relations, said of the emerging guarantees. "But it appears to be ambiguous enough to allow Sharon ... (by) putting a lot of spin on it, to persuade his fellow Likudniks."

The United States is trying to ensure that Sharon's withdrawal doesn't undermine the road map, which has been stuck since its launch last year.

However, Sharon was clearly at odds with the road map when he said Monday he plans to "strengthen and develop" the five settlement blocs Israel intends to keep — Maaleh Adumim, Givat Zeev and the Etzion bloc near Jerusalem, as well as Ariel and Kiryat Arba in the heart of the West Bank.

The road map calls for a freeze in construction in the Israeli settlements ahead of talks on a final peace deal. Both Israel and the Palestinians have violated their commitments, while formally declaring they still abide by the plan. The Palestinians have failed to dismantle violent groups, a key obligation under the road map.

The Palestinian Authority is wary of Sharon's plan. Palestinian officials fear Sharon is sacrificing the Gaza Strip and parts of the West Bank so he can keep other areas.

Palestinians want to establish a state in all of the West Bank and Gaza Strip, and demand Israel remove all settlements from the lands, captured in the 1967 Mideast war.

"If Sharon thinks he can trade off Gaza with maintaining large parts of the West Bank he will just add to the complexities," Palestinian Cabinet minister Saeb Erekat said after Sharon's speech. "Settlements in the West Bank are just as illegal as those in Gaza."

On Monday, Bush held talks with Egyptian President Hosni Mubarak. Bush will also be meeting Jordan's King Abdullah II and Palestinian officials in the coming weeks.