My interest here is in documenting the changes party fortunes in the states and trying to get a sense of which changes are of greatest consequence to the race to 270. Of course, I am not only interested in where Democrats have strengthened their position but also where they have lost ground to Republicans; for there are clearly parts of the country where Democratic prospects have dimmed appreciably over time.
I use state-level presidential election returns from 1972 to 2012 to document the trends in party support. The starting date for this type of analysis is somewhat arbitrary, but using 1972 puts us on this side of the beginning of partisan changes in response to party strategies related to civil rights. I focus on the trend over time in the centered (around the 50-state mean) Democratic share of the two-party vote, separately for each state. Centering the vote allows me to focus on each state relative to all other states without worrying about overall swings in party fortunes from one election to the next. So the focus is not on which party wins or loses a state, but on support for the Democratic party over time in a given state, relative to all other states. To gauge the trend over time I regress vote share on year, separately for each state, and also included dummy variables for presidential and vice-presidential home state advantage, as well as one for southern states in 1976 and 2000. The southern dummy variable is necessary to capture the unnaturally high level of support for the Democratic ticket in the south in response to the candidacy of former Georgia Governor Jimmy Carter. I then used the results from the state-by-state regression models to estimate the trend in Democratic support over time. In doing this I set the values for the dummy variables to zero so the "predictions" reflect the trend over time exclusive of these transitory perturbations.
The figures below document the changes in Democratic fortunes. In each figure, the solid straight line represents the estimated trend in Democratic support over time, and the points represent the actual election outcomes. It is important to recall that in some cases the trend line does not appear to fit the scatter plot as well as it "should" because it excludes the effects of home state advantages and the southern advantages in 1976 and 1980 (see MS and GA as exemplars of this phenomenon). It is also worth pointing out that the observations above the zero point (dashed line) are not necessarily cases in which the Democratic candidate won the state; instead, these are cases in which the Democratic candidate fared better than he did, on average, across the 50 states.
Finally, we have states that were strongly Democratic and became even more so (Massachusetts and Rhode Island), and several states where the change was either very slight and didn't alter the general outlook for either party (Arizona, Georgia, and Mississippi), or that can best be described as flat-liners, impervious to whatever process has driven the changes in other states (Michigan, Oregon, and South Carolina). Technically, there has been some change in this latter group of states but it is so slight that it is barely discernible in these plots.
Of course, not all states have trended so favorably toward the Democrats. The panel of graphs below illustrates the pattern of party change in the remaining states, where Democrats either lost ground to Republicans or just managed to hold their own. One state that stands out here is West Virginia, which has shifted from a place where the Democratic candidate typically ran ahead of his performance across the other forty-nine states to a place that now appears to be a long shot for Democratic candidates. Minnesota is another interesting state. Although the Democratic candidate continues to fare slightly better in Minnesota than in the rest of the country, there has been a very gradual decline in that advantage. Democrats continue to win there, but Minnesota is somewhat more competitive now than it was thirty or forty years ago.
In keeping with data already presented, the dominant patterns over time are purplish states turning blue, followed by fewer purple states turning red, some reddish states turning even redder, and a number of reddish states turning purple.1
The movements over time have clearly favored the Democratic Party. Those states that have moved from somewhat competitive to Democratic, or from leaning Republican to fairly competitive, combine for a total of 227 electoral votes. Among those states that have made similar shifts toward the Republican Party (other than those that were in the Republican column to begin with), the electoral vote count is only 92. Perhaps the worst news for the Republican party is that there is only a single large state that has moved in their direction: Texas, which has gone from leaning slightly Republican to being a strong Republican state. Contrast that with the Democrats, who have seen California, Illinois, New Jersey, and New York move from somewhat competitive to strongly Democratic, and Florida has moved from leaning Republican to very competitive.