RELIGIOUS FACTION SAYS IT WILL QUIT BARAK'S CABINET
Israel's third-largest political party, Shas, says it will quit Prime Min Ehud Barak's government because it does not adequately support programs for poor; party's departure would throw governing coalition into chaos just as it moves into criticial period, facing key budgetary and peacemaking deadlines; Barak asks Shas to wait 24 hours to see what could be resolved, and party agrees; Shas, ultra-Orthodox group, has made so many threats to bolt Barak's coalition that many Israelis, including Barak, are skeptical that announcement is anything more than posturing; Shas's withdrawal would leave Barak with minority government of 51 out of 120 members of Legislature; small parties and Arab parties can be expected to keep Barak's government from falling, but Shas's withdrawal would be destabilizing event at sensitive moment; photo (M)