WASHINGTON , 4 May 2004 — In rejecting Prime Minister Ariel Sharon’s plan to unilaterally withdraw from the Gaza Strip, Israel’s Likud party dealt a blow Sunday to President Bush’s Middle East policy that he could ill-afford after a month of setbacks.

Buffeted by the diplomatic and political fallout of mounting casualties in Iraq and the release of graphic photographs of US soldiers abusing Iraqi prisoners in a notorious Baghdad prison, President Bush badly needed some good news from the region as he heads into a period of Middle East diplomacy.

Administration officials had hoped Bush’s strong public backing for Sharon’s plan — which sparked outrage in the Arab world — would help the prime minister clinch the vote. Instead, Likud members turned their backs on their leader and on Bush, despite Sharon’s dire warnings that US-Israeli relations could be damaged if the plan was rejected. “This is a shocker for the Bush administration,’’ said Shibley Telhami, a University of Maryland scholar and senior fellow at the Saban Center for Middle East Studies at the Brookings Institution, a moderate Washington think tank.

The Arab world will blame the administration, Telhami said, both for backing a policy that was defeated and for having offered concessions that will stand even if Sharon is unable to implement his withdrawal plan. “If there is a big loser in all this, it is the Bush administration — more than Sharon, in a way,’’ Telhami said.

Although he vowed Sunday night to pursue other options, it was unclear that Sharon would be able to implement the Gaza withdrawal without his party’s backing.

“If Sharon fails, if he can’t really deliver, (the administration) is in something of a jam,’’ said Flynt Leverett, a former official at the National Security Council who worked on Israeli-Palestinian issues and is now a Brookings fellow. “It is not clear to me what they will do next.’’

Leverett, a sharp critic of the administration’s Middle East policies, said the administration had been “playing Israeli politics in a potentially dangerous way,’’ by throwing its backing to Sharon when he was in an uphill battle within the Likud. “They’ve antagonized moderate Arab states and Europeans that they need to move things forward, and they’ve damaged their credibility with other parts of the Israeli electorate,’’ he said.

After months of stalemate in Israeli-Palestinian negotiations, Bush administration officials had embraced Sharon’s plan as a possible opening that might change the dynamic between Israelis and Palestinians by convincing Palestinians that the hard-line prime minister was willing to make at least some territorial concessions and abandon some Jewish settlements.

In Israel, it was seen as seriously weakening a leader whom the Bush administration has staunchly supported in his uncompromising war on Palestinian militants and his decision to unilaterally pull out of Gaza and parts of the West Bank. In a statement issued Sunday night in Israel, Sharon thanked Bush for his “deep friendship and leadership.’’

Late Sunday night, the White House, in a statement, said: “We have been informed of the results of the Likud voting. Our own view has not changed: The president welcomed Prime Minister Sharon’s plan to withdraw settlements from Gaza and a part of the West Bank as a courageous and important step toward peace. We will be in consultation with the prime minister and the government of Israel about how to move forward.’’

State Department officials declined to comment.

Bush sparked widespread anger in the Arab world when he offered Israel key concessions last month on territory and the fate of Palestinian refugees in an effort to ensure Sharon’s victory in the Likud vote. For the first time, the United States acknowledged in a written statement that Palestinians who lost their homes when the Jewish state was created would be resettled in a Palestinian state alongside Israel and not inside Israel. The Bush administration also said that it was unrealistic to expect that Israel would fully withdraw from the West Bank in any final settlement. Israel seized the West Bank from Jordan in the 1967 Middle East War.

Many Arabs saw the move as abandonment by the United States of its role as an honest broker and as prejudicing the outcome of negotiations. Jordan’s King Abdallah II, a close US ally in the region, abruptly canceled a scheduled visit to the White House to protest the US policy shift. Palestinians rejected it as an attempt by Israel and the United States to impose a solution on them that would leave wide swaths of the West Bank in Israeli hands and many Jewish settlements intact. Sunday’s vote complicated the Bush administration’s position going into a week of Middle East diplomacy, in which it had planned to promote support for Israel’s withdrawal offer.

United Nations Secretary-General Kofi Annan is hosting a meeting today in New York of the so-called quartet — the international sponsors of the peace plan dubbed the road map that mandates a series of reciprocal steps by Israel and the Palestinians to produce a two-state solution to their conflict. The United States, the European Union, the United Nations and the Russian Federation are in the group.

Secretary of State Colin L. Powell plans to attend the session, along with his counterparts from the European Union and the Russian Federation. And Abdallah is expected to hold his delayed session with Bush next week.

The United States wants backing from the moderate Arab governments as it attempts to stabilize Iraq and prepares for the June 30 handover of sovereignty to Iraqis. But those governments are under growing pressure as fighting continues in Iraq, feeding anti-American sentiment in the region, Telhami said.

Some sort of peace moves by Israel — even if unilateral — might have relieved some of that pressure.