Ex-premier Ehud Barak, trying to regain control of his Labor Party, called on Prime Minister Ehud Olmert to resign by August.
Olmert has lost much of his public support in the wake of the inconclusive war. Barak's resignation call could present a serious challenge to Olmert's hold on power.
Barak, facing political newcomer Ami Ayalon in a runoff election for party leader next week, told a news conference that the inquiry report is "harsh and requires Olmert to draw personal conclusions and resign." In an interim report, the commission faulted Olmert for hasty decisions and unrealistic goals.
Labor is Olmert's main partner in a coalition government. Barak said Olmert must step down by the time the Winograd Commission issues its final report in August. If not, Barak said, if he wins the runoff he will pull his party out of Olmert's government "and work for formation of a new government within the current parliament, or if that fails, work for early elections."
The Labor Party runoff is set for June 12. Defense Minister Amir Peretz, the current party leader, finished third in the first round of primary voting last week and was eliminated. He has pledged his support to Ayalon, former head of the Shin-Bet security service and commander of Israel's navy. Ayalon favors pulling Labor out of Olmert's government.
Up to now Barak, who served as chief of staff of the Israeli military before he was premier, said he would serve in Olmert's Cabinet, presumably as defense minister, for a limited time, but he did not set a deadline.
There is also internal pressure in Olmert from his Kadima Party to step aside. Popular Foreign Minister Tzipi Livni said Olmert should resign because of the commission report, and she is a likely candidate to replace him.
Under Israeli law, if Olmert steps down, the president would pick another member of parliament to form a government. Failure would result in a new election. Olmert was elected in March to a four-year term, but Israeli governments have been falling every year or two for much of the last decade.