BEIRUT, Lebanon, March 18 – Political leaders trying to form a new government in the wake of the past month’s upheavals said today that negotiations had stalled, raising the possibility that nationwide elections scheduled for this spring might have to be postponed. Allies of the pro-Syrian government and the opposition said the two sides disagreed on a number of important issues, including a demand by opposition leaders for an internationally supervised investigation into the murder of Rafik Hariri, who was killed by a car bomb Feb. 14.Opposition leaders, who have been galvanized by a huge outpouring of public support since Mr. Hariri’s death, said they were still waiting for a response to a list of demands they presented two days ago to the caretaker government of Omar Karami. Mr. Karami, leader of the pro-Syrian government, resigned under pressure earlier this month but was asked to return by the Parliament.