Campaign strategists are gearing up to flood Facebook with inflammatory and polarizing ads heading into the midterm elections, saying its advertising platform rewards extreme messaging more than other venues.

Such tactics often backfire, partly because extreme ads can be hidden by those who find them disagreeable, Facebook Inc. said. The disconnect highlights the social network’s often-controversial role as a political tool, where Facebook wants to be viewed as a promoter of healthy debate while some believe its platform contributes...