In order to measure the strength of the parties in each state, the Major Party Index (MPI) was built by averaging the results of the six major elections that take place in the fifty states. This index allows us to describe the absolute and comparative partisan leaning of each state in each election and identify trends of party strength over time within individual states, among regions, and within the nation as a whole. The MPI sheds considerable light on three general developments: (1) a national change from Democratic dominance in the 1980s to a Republican edge by 2002, (2) significant regional realignments in the South and New England, and (3) a strong trend toward greater consistency between partisan voting at the federal and state levels.
Party strength in the United States is generally measured by public opinion polls in which respondents report their own partisan identification. There are well-known advantages to this approach. It permits analyses that relate an individual's party preference to attitudinal and demographic factors, thereby allowing for the generation and testing of hypotheses about the causes of party attachment. Self-identification can also be correlated with reported votes, which allows for inquiries into the relative importance of party preference for electoral behavior. For these reasons, poll surveys have become the preferred standard in academic research. But this method has admitted weaknesses. Polls are expensive and they often have a significant margin of error, which in the case of general national polls increases greatly as one begins to investigate particular sub-groups or geographical regions. Partisan self-identification by itself, moreover, does not tell us how people actually vote. For example, many white Southerners after 1950 identified themselves as Democrats or independents even as they had become reliable Republican voters at the presidential level. Finally, partisan self-identification itself says nothing about the habitual behavior-if it existsof independents. Many analysts today suspect that most voters who call themselves independents in fact have a fairly distinct partisan leaning when it comes to casting their ballots. '
Another approach to assessing party strength is to look at election results and measure party standing by the votes each party's candidates received. This method also has its strong points. It is inexpensive (at least for scholars), as the government picks up all of the costs of the research project by holding elections. The margin of error is fairly low, with the inaccuracies being limited to fraud and the now welldocumented difficulties encountered in counting and recording voles (hanging chads, undervotcs, voting machine failure, and the like).2 Election results also have the advantage of directly measuring the phenomenon: if one is interested in party strength, it makes great sense to look at how citizens actually vote. Still, there are weaknesses in using electoral results. The statistics are tied to aggregates (collective units), not to individuals, which makes it impossible to connect the vote to attitudinal variables and difficult to relate it to many demographic factors (other, of course, than geography itself).3 Another problem is that a vote for a particular candidate is by no means the same thing as an expression of support for a party; it may only reflect a preference for a particular individual, as when millions of Democrats pulled the lever for the war hero Dwight Eisenhower in 1952 and 1956. Election results from any single election may therefore reveal little about "real" or "normal" party strength.
All approaches are bound to have strengths and weaknesses, and the question of a measurements worth should be judged on the practical grounds of its helpfulness as an investigative tool. Despite the drawbacks just noted, the idea of measuring party strength on the basis of election results seems attractive enough to hold considerable interest for political scientists-not as a substitute for, but as a supplement to, polling methods. …