National Public Radio (NPR) has created a fascinating video showing the electoral geography and electoral advertising money being spent.
In short, the country is the United States of Purple (purple being a mixture of Republican red and Democrat blue). These states, which demographically and cultural are a mixture of America, will decide the presidential election as the base Blue and Red states are not enough for either party to win out right. The Purple states (Nevada, Colorado, Minnesota, Iowa, Wisconsin, Michigan, Ohio, Pennsylvania, New Hampshire, Virginia, North Carolina, and Florida) comprise 156 electoral votes (29% of all electoral votes/58% of the needed 270 to win). However, by watching the video one can only assume 90%-plus of all campaign cash is being spent in these states.