I've long maintained that part of the reason that the Republican party was successful in the 80s and 90s was because we purged the wackos (the Klan, the neo-Nazis, et al) from the party in the 70s and early 80s. The Democrats embraced their wackos (the communists, the hard greens, etc) and now it shows. Most of the country disagrees, violently, with what the hard left wing of the Democrat party believes. If they fail to purge their extremists, a good chunk of the middle will move to the Republican party, not because the middle suddenly became conservative, but because the Republican party would offer the only voice of sanity.
I can see three de facto parties emerging thusly: A hard left seriously wacko party (10-20%), a moderate party (40-50%) and a conservative party (30-40%). They may still be under the names "Democrat" and "Republican," but it'll be three parties nonetheless. We're already seeing the seeds of that, with the Blue Dog Democrats and the Tea Party movements.
John / Billybob