The Democratic party has always been fissiparous but bound together somewhat artificially by the conviction that whatever their opponents hold must be wrong. That has devolved somewhat into the politics of reflex opposition that is the hallmark of the perennial loser, curious because for eight interminable Clinton years they weren't, at least in the executive. They are now.
I could easily envision either party or both breaking up. The Republican/Democratic party dichotomy these days cuts across several watershed issues - gun control, abortion, national defense, national sovereignty, nationalism/internationalism - that would normally represent the real dichotomy within the American body politic. It is a healthy thing that members of one party lean toward the positions of the other party on some of these but it isn't necessarily a stable one. Whatever - stability here may not be a virtue. God bless America and hang on for the ride, my friend!
I'm occasionally tempted to write an essay about the coming splintering of the Republican Party, particularly right now when the Miers appointment exposes the major fault lines in the party. And I suspect that split will occur, but not until the Democrats crack up first.
I could see an unstable world with five significant political parties.
Ironically, this is what Madison had in mind with the Constitution. Neither he nor Hamilton envisioned British-style political parties taking root here, and Washington opposed the establishment of parties with every fiber of his being. (The various factions in the cabinet and Congress waited until Washington's body was cold at least a few hours before they openly formed parties.) Madison's construct was designed to handle a large group of factions, and the more factions the merrier. This could be where we're heading.
At least Madison gave us the tools to handle this scernario.