We mostly hear about a handful of swing states these days, as presidential campaigns focused more on optimizing resources. Research by Shaw et al. identifies the shifts and the state-by-state results. For the Washington Post, Hannah Dormido and Adrián Blanco show the changes as a set of maps over time.
In the 1950s, you see wide coverage between the Democratic and Republican parties. In the 2000s, the strategies grow more sparse. I suspect the internet and an increasing volume of data had something to do with that.
