Suppose I don't?
Suppose I don't?and Supposing I don't?
Inf. And what will happen if I don't? (Said as a sort of threat. Not usually with question intonation.) Bill: You'd better get yourself over to the main office. Tom: Suppose I don't? Father: You simply must do better in school. Tom: Supposing I don't? Father: Your clothing and personal belongings will be placed on the curb for the garbage pickup, and we will have the locks changed. Next question.
See also: suppose