Perhaps this is me disagreeing with the political implications of the use of "ploy", and could be due to my political science background.
Yes, every political campaign strategizes and takes actions based on strategy. Demographical benefits, be it due to geography, class, race, sex, religion, age, interests, etc are definitely taken into consideration. To not do so would be foolhearty. However, to me, to suggest that a pick is a "ploy" specifically to win over a specific voting block it implies that the individual is chosen singularly due to that particular characteristic and that otherwise they would have no use in being picked.
Looking at Biden for example...
Does the fact that you're choosing a "White" guy perhaps help a bit with demographics? Sure. However, I wouldn't call his choice a "ploy" to get the white vote because there are far more, and some better, reasons to go with him. Equal to the "white" guy was the fact he's viewed as somewhat of a "rural" and "Country" guy, with Pennsylvania where he grew up being viewed more as a "rural" country area than a "north eastern" type of location. Biden also chaired the Senate committee on foreign relations, shoring up a weak point in Obama's experience portfolio. Speaking of experience, that was another weak spot of Obama's. Biden has over 30 years as Senator. There were numerous reasons why Biden made a good running mate to pick for Obama beyond his race, and while it may've and most likely did play into the consideration I think to call it a "ploy" for the white vote to be a gross over statement.