Mich. Comp. Laws § 168.107: United States senator; vacancy, creation.

Sec. 107.

The office of United States senator shall become vacant on the happening of any of the following events before the expiration of the term of such office: The death of the incumbent; his resignation; his removal from office; the decision of a competent tribunal declaring void his election or appointment; or his refusal or neglect to take his oath of office.