Mich. Comp. Laws § 168.92

Sec. 92. A general primary election of all political parties shall be held in every election precinct in this state on the Tuesday succeeding the first Monday in August preceding every general November election, at which time the qualified and registered electors of each political party may vote for a party candidate for the office of United States senator, to be filled at said election: Provided, That this section shall not apply to parties required to nominate candidates at caucuses or conventions: Provided further, That no nomination for the office of United States senator shall be made unless such official is to be elected at the next succeeding general November election.