Rep. Mike Castle's (R-DE) defeat Tuesday night for the GOP Senate nomination in Delaware might be signaling the end of an era in Delaware politics.

When Castle became governor of Delaware in the mid-1980s, he was depicted as being the hand-selected choice of former Delaware Gov. Pete du Pont (R).

In 1992, Castle transitioned from the governor's mansion to the U.S. House. There was then what was famously called in Delaware as "the swap." That's where then-Rep. Tom Carper (D-DE) ran for governor when Castle ran for the House. Then Carper moved on to the Senate.

A well-known observer of Delaware politics named Celia Cohen referred to this as the "age of incumbency." Cohen argued that leaders of both parties were always being re-elected, or just toggling around to other positions.

It could be said that with O'Donnell's victory, the age of incumbency is over in Delaware.