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Mayhew. 1974. Congressional elections: The case of the vanishing marginals. Polity 6:295-317.

A New Puzzle: The Vanishing Marginals

Mayhew observes that a rapidly decreasing number of incumbents win by a small margin. The margins have been growing larger. Thus, the incumbency advantage has grown rapidly as well. Note, in Figure 10.1, how the graphs in the left column change from being normal to bimodal. Thus, whereas most elections were once won with the Democratic candidate getting around 50% of the vote, there are now two modes: The Democrat either wins around 35-40 or around 60-65. At the same time, the same districts have remained competitive in presidential and open-seat elections--so the districts themselves aren't changing, incumbency is.

Five Possible Explanations

Mayhew's goal is to bring this puzzle to our attention, not to solve it. Nonetheless, he proffers five potential explanations and discusses each briefly.

1. Redistricting: Probably not. If this were true, then we would also see district-level presidential returns becoming bimodal--but they aren't.
2. House members advertise themselves better now. (Evidence: Dramatic increase in franked mail.)
3. House members are getting "more political mileage out of federal prorams" (like housing grants, education grants, anti-pollution grants)
4. House members are becoming better at position taking. (i.e. using better public opinion polls to see what the district wants).
5. As voters grow less satisfied with parties (as voting cues), they may be paying more attention to other cues (like incumbency).

SEE ALSO notes under Weisberg (1999), part III.