Congressional Democratic leaders today ordered a formal investigation into whether Ronald Reagan's Presidential campaign made a deal with Iran in 1980 to delay the release of American hostages until after the election. Thomas S. Foley, the House Speaker, and George J. Mitchell, the Senate Democratic leader, said they had designated the House Foreign Affairs Committee and the Senate Foreign Relations Committee to supervise separate but coordinated inquiries. "These allegations are both persist ent and disturbing," the two leaders said in a joint statement. "We have no conclusive evidence of wrongdoing, but the seriousness of the allegations, and the weight of...