That's the intriguing question being asked by Charles Mahtesian, the editor of the Almanac of American Politics, in the current National Journal:

Congressional party-switching has been an almost exclusively Democratic malady for more than a quarter-century. When the occasional Republican like Jeffords or then-Sen. Bob Smith of New Hampshire did leave the family, the defector became an independent, never going so far as to formally join the Democratic ranks. (Smith, in fact, returned to the GOP fold in November 1999, less than a year after leaving it for a short-lived presidential bid.)

However, if the Democrats retake the House this November 7, the self-serving calculus used by a generation of Southern politicians in defecting from the Democratic Party may well begin to make sense for nail-biting, blue-state Republicans across the Northeast and in parts of the Midwest as they begin to ponder a future without chairmanships, a future weighed down by the drag of a socially conservative, Southern- and Western-based national party.