Prior to an attack, if a legislator fails to designate the required minimum number of emergency interim successors within thirty (30) days following the effective date of the act or, after such period, if for any reason the number of emergency interim successors for any legislator falls below the required minimum and remains below such minimum for a period of thirty (30) days, then the party leader of the same political party in the same house as such legislator shall promptly designate as many emergency interim successors as are required to achieve such minimum number. But the party leader shall not assign to any of his designees a rank in order of succession higher than that of any remaining emergency interim successor previously designated by a legislator for succession to his own powers and duties. Each emergency interim successor designated by a party leader shall serve at the pleasure of the person designating him, but the legislator for whom the emergency interim successor is designated or any subsequent incumbent of his office may change the rank in order of succession or replace at his pleasure any emergency interim successor so designated.