Something similiar did happen back in 1994. The Clinton administration, along with the Democratic-controlled House and Senate, ended up alienating many a conservative Democrat.
This process allowed the GOP to get a very strong foothold in the South, and even added a couple of high profile folks to the GOP ranks. Senators Ben Nighthorse-Campbell (D-CO) and Richard Shelby (D-AL) ended up switching over to the GOP as a result of this split.
2006 was the first year in a while, that the Democratic Party actually reached out to many of those conservative Democrats that they essentially cast aside a dozen years before that. In doing so, they were able to pull off several victories that knocked off some high ranked players in the GOP.
For example, the conservative Robert Casey, who actually had the NRA's endorsement back in the earlier part of this decade, defeated the #3 ranked GOP senator, Rick Santorum. With the NRA remaining pretty much neutral in this race, that allowed him to eke out a victory.
The conservative John Tester beat out incumbent GOP senator Conrad Burns in Montana. In a state that's been a solid GOP stronghold, this was quite a feat.
Jim Webb knocked off George Allen in Virginia, in a similar manner.
If the Democratic Party is to maintain its majority, then it can't afford a similar type of splitting.