COLUMBUS, Ohio (AP) — Democratic presidential nominee John Kerry canceled a campaign visit to Columbus on Thursday because of a strained voice, a campaign spokeswoman said Wednesday night. "With the debates coming up, he needs to balance his campaign schedule and keep his voice strong," spokeswoman Kathy Roeder said. [snip]Kerry also canceled a trip to Iowa on Thursday to rest his voice, and the campaign said vice presidential nominee John Edwards would take his place.