1. THE CHANGING BATTLEGROUND

2. Battleground States
Battleground states usually defined as states in which the Presidential race is closely contested, in which the winner gets no more than (say) 53% of the two party vote.
This definition works well in elections which are expected to be close, e.g., 2000 and 2004.
Here I define a battleground states as those whose popular vote split deviates no more than ±3% from the midpoint of the popular vote range in which pivotal state = marginal state.
In practice, this is just about the same as the states whose popular vote split deviates no more than ±3% from the national popular vote split.

3. Battleground States (cont.)
It is of interest to identify battleground states over the last several decades and see
whether the battleground has expanded or contracted over time;
whether the same states tend to have battleground status in election after election;
whether battlegrounds tend to be concentrated in particular regions.