Sharon Warns of Snap Elections Without Likud Backing  JERUSALEM (Reuters) - Israeli Prime Minister Ariel Sharon  urged his Likud party on Thursday to vote for a governing  alliance with the Labour Party, warning that failure to do so  would force snap elections and derail his Gaza withdrawal plan.  Internal polls predict victory for Sharon in a crucial ballot  by the rightist Likud's Central Committee on whether to allow  him to open talks with center-left Labour on joining a unity  coalition to shore up his shattered government.