Senators who seek re-election usually achieve it. Yet it is not clear whether they tend to win mainly because they are incumbents or because their party is strong in their states. Thus, the two principal questions motivating this study are the following: (1) What is the relative importance of party and of incumbency in influencing the outcomes of Senate elections? (2) How has their relative importance changed over the last quarter-century? To answer these questions, a theory is developed accounting for the outcomes of Senate elections in terms of the major potential sources of electoral support given to the candidates, namely, party loyalty, incumbency, “national tides,” and idiosyncratic factors (e.g., issues, personality, local conditions). The theory is then represented in a formal model for which are generated multiple regression estimates of the respective roles of party and incumbency in all postwar Senate contests.
The major finding is that the relative importance of party and incumbency has changed dramatically over the last quarter-century. Party has undergone an overall decline in influence, while incumbency has experienced a roughly proportionate increase. At the same time, the importance of idiosyncratic factors has grown. The implications of these results for broader theories of American politics, including the argument that the United States has been experiencing a “critical realignment,” are noted.