A New York Times story yesterday pointed out that presidential candidates have appeared in only ten states since the conventions, while John F. Kennedy campaigned in 49 states and Richard Nixon campaigned in all fifty during the 1960 election. The story said that this change reflected a decline in the number of states that are close enough to be worth fighting for and cited a few examples. I decided to look more systematically, using all presidential elections from 1916-2008. The figures show the interquartile range--the difference in percentage of the two-party vote between the states with fairly high (75th percentile) and fairly low (25th percentile) Democratic shares of the vote. I show two figures, one for all states and one excluding the South.
By both measures, the gap has been rising since the 1970s--that is, Democratic states are more Democratic and Republican states are more Republican. Of course, even in 1960 both candidates had states that were out of reach, so their decision to campaign in all of them must have reflected some kind of convention or sense of obligation rather than a real expectation that they could win. But it does seem like the states are pulling farther apart, leaving few close ones.
PS. The unusually high figure for 1924 occurred because there was a significant third-party candidate, Robert La Follette (Progressive), who pulled a lot of votes from the Democrats in the Midwest and West. For example, in California the Democratic candidate got only 8% of the vote, vs. 57% for the Republican (Calvin Coolidge), and 35% for La Follette. There were 18 states in which the Democrats received less than 30% of the two-party vote.