Problem being that there's always the specter of total dominance by a united (try not to laugh) Democratic Party. It's the same thing that holds the Dem centrists and wannabe Greens together. Maybe if both parties agreed to split simultaneously, and we'd have a 4-party system of center-left/center/center-right/moonbat.
That's certainly a threat, but they're a threat to each other as well. The wingnuts see the rest of the party as weakling half-liberals who are holding back the One True Cause and must be expunged, where as the rest of the GOP are terrified by the wingnuts and their crazy ruining every attempt at bipartisanship they begin.
The moderates and leftists of the Democrats, on the other hand, are not particularly threatened by each other and are skilled at cooperating. They have nothing to gain by splitting, while the non-wingnut GOP has a chance at getting some of their goals completed through bipartisanship if they drop the wingnuts.
I suppose my minor quibble there is that the mainstream GOP isn't upset at the fringe ruining their attempts at bipartisanship, but rather for constraining their tactical options, which makes it easier for them to fall prey to a well-planned political trap and means they can't take advantage of opportunities. Boehner could have given up the tax hike on $250,000 after token resistance, and then put the onus on the Democrats to match that "huge" sacrifice with more draconian cuts than they were comfortable with. But he never could have sold that to the Tea Party.
It's like a giant game of Magic: The Gathering, and the Republican side has a card in play that never lets them sacrifice one of their own cards, even if it would work to their benefit. (Geekiest. Political Analogy. EVER.)