Israeli Prime Minister Ariel Sharon called for a referendum within his Likud Party on his plan to unilaterally withdraw from the Gaza Strip and parts of the West Bank, Israeli officials said Friday. Sharon's request, lodged with the party Thursday night, means that Likud will likely hold its vote on April 29, Israel TV reported Friday. Sharon had initially said he would not call for the referendum until after he returned from a trip next week to Washington, where he will meet with President in an effort to garner American backing for his plan.