THREE MONTHS after its unilateral withdrawal from the Gaza Strip remade the landscape of the Middle East, another groundbreaking pullout has reshaped Israel's domestic politics. In this case, it is the departure of Prime Minister Ariel Sharon from the right-wing Likud Party, which he helped found in 1973 and which has led the Israeli government for 20 of the past 28 years. Mr. Sharon's new National Responsibility Party separates loyalists of the Israeli leader from Likud members who sought to undermine him and stop the retreat from Gaza; the prime minister is now positioned as the centrist candidate in elections to be held in March. Together with the Gaza withdrawal, this political upheaval could pave the way for an Israeli-Palestinian settlement, or another long period of stalemate and violence. Much will depend on the unpredictable decisions of Israeli voters -- and the equally unpredictable Mr. Sharon.

The latest in a lifetime of bold Sharon gambits followed a momentous internal election by Israel's Labor Party, which ousted venerable leader Shimon Peres, Mr. Sharon's partner in a coalition government, in favor of a 54-year-old trade union leader, Amir Peretz. He quickly withdrew his party from Mr. Sharon's government; his supporters believe he has the opportunity to lead Labor back to power by focusing on the inequalities created by Likud's embrace of free-market economics. That move forced Mr. Sharon's hand; though popular among Israelis generally, he has faced bitter resistance within Likud both from political rivals and from die-hard supporters of the Jewish settlement movement in Gaza and the West Bank. The split brought him immediate benefits: About one-third of Likud's parliamentary deputies defected with him, and initial polls showed that Mr. Sharon would defeat both Likud and Labor in a general election.

It remains to be seen whether Israelis really will embrace the new party on an election day four months away. Even murkier are Mr. Sharon's intentions: So far, he has offered few indications of what his party will stand for. Some speculated that the prime minister mainly sought to rid himself of troublesome Likud adversaries and consolidate presidential-like authority in a new government; he told reporters that "life in the Likud has become unbearable." Others suggested that Mr. Sharon made his move so as to enable a decisive follow-up to Gaza. At the first meeting of his new party, he said he would seek "to lay the foundation for a peace in which we set the permanent borders of the state, while insisting on the dismantling of the [Palestinian] terror organizations." In nearly four years in office, Mr. Sharon has preferred unilateral measures to negotiations with Palestinian leaders; does he mean to set "permanent borders" by peace accord, or fiat? The Bush administration, and others with vital interests in the Middle East, will have to wait and see.