jfern, you will never have anything approaching a filibuster-proof majority without a big tent. That's one of the problems the GOP has had recently - the Reagan coalition included a lot of people from western and northeastern states that didn't exactly qualify as "far-right".
A *lot* of the Democrats in Congress, and even a couple in the Senate, are Dixiecrats or similar. If the Democratic party ostracizes them, then the only surviving rural Democrats will be representatives from overwhelmingly African-American districts, or from a few states with left-leaning rural areas (like the Upper Midwest and Northern New England). If a liberal Democrat runs in that district, then it will most likely go Republican.
And it isn't just Congress. Consider the governor of Tennessee, Phil Bredesen. Not exactly a friend to the far left (which is why I think he's the best Democrat Obama could choose for HHS
). If the Democrats try to knock him out in favor of a "Real Democrat", and the Republicans run someone more competent than a pineapple, the Republican will probably win.
