ATHENS — In a bid to consolidate his political support and complete a crucial bailout with creditors, Greek Prime Minister Alexis Tsipras on Thursday called for a vote within his party to decisively sideline rebels opposed to the rescue deal. The radical-left Syriza party members would be asked in the vote, scheduled for Sunday, whether they accept his plans to reach a deal on a package of loans from creditors in exchange for more tax increases and budget savings. Tsipras is gambling that with a win, he would neutralize the rebel party members, a group who contend the terms are too painful and that Greece would be better off leaving the euro.