Incumbents have always fared well against challengers. Indeed, it would be surprising if those in office did not do better on average than those unelected. The important matter is that this advantage has reportedly increased in recent decades. Numerous studies indicate that beginning in the 1960s, incumbents were able to win more frequently and increase their vote percentages. Incumbents have always had a high success rate versus challengers, and now they do even better.

Specific trends will be examined later, but several changes indicate how members of Congress have been able to change the electoral landscape. Members are now able to increase their vote percentages from their first to their second elections more than in the past. The average percentage of the vote incumbents receive is now greater than in the 1940s and 1950s. More incumbents are elected with more than 60 percent of the vote, a common hurdle to achieve a safe seat. Members of the House are now able to stay in office longer than in the early 1900s. From the 1940s through the 1970s, the correlation of their electoral vote to the presidential vote in their district declined, reducing the threat of loss from a national swing in sentiment against one party. They were able to disconnect their vote from the president's vote. It became increasingly common for scholars and commentators to note that the incumbency effect was powerful and growing.