Incumbency Advantage

Members of the House who seek reelection have won more than 90 percent of the time in nearly every election since the mid-twentieth century, often around 95 percent, and senators somewhat less often. In 1974 political scientist David Mayhew described the vanishing marginals, the decline in closely contested House seats, and his book Congress: The Electoral Connection argued that members organize their work around reelection through advertising, credit claiming, and position taking.

Sources of the advantage include name recognition, free mail and communications to constituents, staff who handle casework for voters, the ability to claim credit for federal projects, media access, and a strong fundraising edge, since donors and political action committees favor likely winners. Incumbency also discourages strong challengers from running, a scare-off effect. Redistricting can help by drawing safe seats, though most incumbents who lose do so in wave elections, such as 1994 and 2010, or after a scandal.

Measured as the extra share of the vote incumbents win, the advantage grew from a few points in the 1950s to as much as 8 to 10 points in the 1980s, then declined as elections became more nationalized and partisan, with voters increasingly choosing by party rather than by candidate. Defenders say incumbency rewards experience and responsiveness to constituents. Critics say it entrenches officeholders, limits competition, and reduces accountability, arguments often made for congressional term limits.