The governor may proclaim martial law when the public safety requires it in case of rebellion or actual or imminent invasion, and may order all or any part of the organized militia into active state service to enforce the proclamation. The militia shall assume only those functions of civil government specified by the governor, or those that, in the discretion of the militia commander, must be assumed in order to accomplish a specific mission assigned by the governor. Martial law may not continue for longer than 20 days without the approval of a majority of the members of the legislature in joint session.