By the 1980s presidential and House results were beginning to come together. The correlation between results was low during the 1960s and 1970s, but in 1980 the association began to increase, and by 2008 it was close to the level that prevailed in the early 1900s. Why and how did that happen? If results were converging, what was happening to incumbents?

The simple answer as to why the presidential–House correlation increased is that the number of split-district outcomes declined. A split district is one in which the presidential and House winners differ. The difference that creates this split could be small. It is possible that a presidential candidate might lose a district with 49 percent and the House candidate could win the district with 51 percent of the vote, creating a difference of two percentage points.

Although the difference might be small, over the last century split-outcome districts have averaged much larger deviations of House results from presidential results than for non-split districts. Figure 10.1 indicates the average absolute value deviation of House results from presidential results for split and non-split districts. From 1900 through 1916 the differences between the two types of districts were minimal. Beginning in 1920 and continuing through 1988 the average deviation increased significantly, rising from somewhat over 10 to the high 20s. Then in 1992 it declined, and has fluctuated between 15 and 20 since then. For non-split districts the deviations of House results from presidential results has been relatively constant and much lower.