Mich. Comp. Laws § 168.99
When the candidate of a political party, after having been nominated to the office of United States senator, shall die, withdraw, remove from the state, or become disqualified for any reason, the state central committee of such party shall meet forthwith and by a majority vote of the members thereof shall select a candidate to fill the vacancy thereby caused. The name of the candidate so selected shall be immediately certified by the chairman and the secretary of said committee to the secretary of state and to the board of election commissioners for each county, whose duty it is to prepare the official ballots; and said boards shall cause to be printed or placed upon such ballots in the proper place the name of the candidate so selected and certified to fill such vacancy.
MCL 168.99