In 1974 David Mayhew (1974a) called attention to a significant change in House election results. He classified districts as safe if the incumbent won with 60 percent or more of the vote and marginal if the incumbent received less than that. He then compared the frequency of safe and marginal districts for the years 1946–1972. It was clear that the percentage of marginal districts decreased in 1966. Figure 2.1 shows the percentage of House elections involving incumbents who had marginal outcomes from 1956 to 1972. Something happened in 1966 and in subsequent elections involving incumbents. For 1956–1964 an average of 40.4 percent of House incumbent outcomes were marginal. For 1966–1972 the average was 29.7. Subsequent studies confirmed the existence of this trend (Cover and Mayhew, 1977: 63; Krehbiel and Wright, 1983: 143).

Mayhew's speculative, but very plausible, explanation of this change was that members were allocating themselves more of the resources that could be used to increase their visibility, popularity, and vote percentages. Members were sending out more government-funded mail to constituents, allowing them to boost their visibility. They were performing more constituency services, helping constituents with problems. The number of grant-in-aid programs was increasing, allowing members to claim more credit for bringing benefits to the district. They were doing more for their constituents and had more resources to advertise these efforts (Mayhew, 1974a: 310–11, 1974b: 53–60, 84–85).