JERUSALEM — Ariel Sharon's efforts to put together a unity government were on shaky ground this week as the Labor Party found itself in total disarray.

On Tuesday, Prime Minister Ehud Barak was virtually forced by his own party to resign from Labor and decline Sharon's offer to become defense minister.

Barak lost the support of many of his Labor colleagues after Sharon beat him in a landslide vote for prime minister. Those Laborites said they would not support a unity government as long as Barak was involved.

With Barak gone, the Labor Party was left leaderless, at least until Monday when an election for a new head is scheduled.

Meanwhile, Sharon set up meetings with Labor members in the hope of keeping his dream of a unity government alive.

Sharon reportedly offered the defense ministry to former Prime Minister Shimon Peres. Peres has been in line for the less important foreign ministry.

Two other leading Labor politicians, Communications Minister Binyamin Ben-Eliezer and Deputy Defense Minister Ephraim Sneh, said they, too, covet the defense post under Sharon.

But Barak supporters within Labor still hoped to have the last say. Labor Knesset member Weizman Shiri pledged he and other Barak loyalists now would do all they could to foil a unity government or, failing that, to prevent party members who felled Barak from serving as ministers.

Barak, meanwhile, blamed Sharon for scuttling the agreement the two had made for a unity government.

In a bitter letter Tuesday night, Barak said the spirit of unity had been shattered when Sharon made it clear he expected to decide policy autocratically.

Barak referred obliquely to Sharon's determination to bring far-right politicians into his government, in defiance of Barak's demand that those groups be kept out if Labor joins.

However, political observers say the main reason Barak decided to quit was the mounting wave of revulsion — in Labor and the country at large — when he backtracked from his earlier resignation on election-night.

During his term, Barak was accused of constant policy shifts that undermined his credibility. Just days after announcing his resignation on Feb. 6, Barak insisted on running Labor's unity negotiations with Likud. It soon emerged that Barak intended to lead Labor into the unity government and take the No. 2 spot of defense minister.

Barak argued the move was mandated by the national interest, even though his own personal and political interests would be better served by retiring from political life for now.

Open rebellion soon erupted in Labor. Major party figures spoke out publicly against Barak's "zigzagging."

Interior Minister Haim Ramon, who favors unity, went to Sharon's ranch in the Negev to state he would only join a unity government if it didn't include Barak.

On Tuesday, the beleaguered Barak closeted himself with his closest political advisers, and by late evening it was clear a resignation was in the works.

Midweek, Labor Party Secretary-General Ra'anan Cohen said he would consult with Peres and other key figures before submitting a slate of ministers to Labor's Central Committee. Likud has said defense, foreign affairs and five other portfolios are available for Labor.

In his letter to Sharon, Barak wrote he still favors the unity option in principle, provided a way can be found to collaborate in making policy. Labor's central committee is likely to endorse the option if acceptable terms can be negotiated.

However, key figures in Labor's dovish wing, including Justice Minister Yossi Beilin and Foreign Minister Shlomo Ben-Ami, are implacably opposed to such a step. They argue Labor would serve as little more than a fig leaf for Sharon's allegedly anti-peace policies.

Labor's participation in the government, they say, would blur the distinction between the two large parties, and weaken Labor's ability to make its case to the voters in the next election.

They further argue leftist voters would abandon Labor for the more dovish Meretz Party, which has declined Sharon's invitation to join a unity government.

As to the leadership battle, Knesset Speaker Avraham Burg and Ben-Eliezer already have declared, while Ramon and Ben-Ami are weighing their prospects.