Why are swing states important? (Brennan Hoban, Brookings)

Thirty-three states have voted for the same party in the past five presidential elections, and 40 of the 50 states have voted for the same party since 2000. Predictability like this means that many states can essentially be written off, far in advance of Election Day, as a sure win by one or the other candidate. The remaining battleground states, or swing states, are the focus of presidential campaigns. Seventy-five percent or more of a presidential candidate’s spending occurs in these key states.

https://www.brookings.edu/blog/fixgov/2016/09/28/why-are-swing-states-important/