Not anyone. If you ran in a primary and lost, you cannot be a write in. So a write in would have to be a new name. Kit Bond has been mentioned.
I don’t know the political landscape of Missouri but I am about to embark on that adventure. Do they have a Republican Governor, and if so, does he/she have the power to appoint a Senate seat left open after a potential victory over McCaskill?
What I am getting to is that here in Florida, Martinez quit with over a year left on his term. The Governor (Crist) appointed his buddy Peppi LeMieux to finish out his term. What if the party did a write in like Bond, Bond wins and shortly thereafter decides to resign for health reasons and the Governor appoints the runner up Brunner?