The major change regarding incumbents during the years 1946–2006 has been the growing electoral security of Republicans. Realignment and the gradual shift in the electoral bases of the parties are central to this change. As these shifts have occurred, led largely by changes in the electoral pursuits of presidential candidates, many incumbents have found themselves left behind by their parties. Republicans, in particular, have struggled with this, and many incumbents in the party have run in districts where their presidential candidates were not doing well, holding down their margin. Eventually, the party's out of synch incumbents retired and were replaced with incumbents in districts where Republican presidential candidates did well. These newer Republicans, running in districts more amenable to Republican ideas, won higher percentages, resulting in more safe Republicans.

The Republican changes have involved remarkable shifts in the electoral bases of their presidential candidates. In the early 1900s Republican presidential candidates did very well in New England and the remainder of the North and poorly in the South. The aftermath of the Great Depression left them weakened in the first two areas and with very little support in the South. By the 2000s, Republican support has completely eroded in New England and is very strong in the South. The parties have essentially exchanged their areas of greatest strength over time (Stonecash, 2005); that is, what was strongly Democratic has become strongly Republican, and what was strongly Republican is now strongly Democratic.