If you're talking about a formal break-up, or anything more than a few percentage points worth of Democrats absconding to different parties, don't count on it. The party may splinter, may have already splintered into different factions fighting for control of the same party, but never forget that the Democratic party survived a civil war and reconstruction. Though the Democratic party hasn't been around since Thomas Jefferson, as they claim, they have been around as an organization since Andrew Jackson. The party's current circumstances are probably just more water under the bridge.
You may be right, the party's inerta may be just too great for it to fragment so completely.
However, what the issues and constituencies the Democratic Party "fights for" are so radicaly different than in the era of the Civil War and the Reconstruction, does it really bear any resemblance to the party that weathered those "storms"?
IMO, the closest historical continuity the DNC has is to the socialist era of FDR and the "new deal". But that's just the window dressing. In reality the Democrats are a loose coalition of Unions, Gays, Minorities, Socialists, Feminists, Environmentalists, and FDR/JFK-era "just because" knee-jerk Democrats.
Blue-collar unions (not counting civil service) are dwindling. They also trend socialy conservative on everything but labor law.
Gays probably have the the broadest interest in the various platforms, but I still see a disconnect with blue-collar interests.
Minority communities are traditionaly very homophobic. There is also the growing wedge issue in the cities of school-choice/vouchers. There is a smaller subcurrent of the DNC "taking them for granted" that is slowly growing.
The hard-core Socialist-leftist wing of the party is starting to go on purges, they can't stand the moderates in their midst. Lieberman is but the latest victim.
The (Gender, as opposed to merly "equality") Feminist movment is dying. There are legions of women who are thankful for the advances, but don't want to be reviled for being religious, or a stay-at-home-mom should they choose. The vangaurd of the feminist movment is now career women who are 40- 50, unmarried, and childless& and scaring the hell out of younger women who realize "having it all" really means "having half".
The Environmentalists can barely stand the DNC now as it is. Their interests diverge from the Union wing almost constantly. Since the DNC does "play ball" with industry, they hate them. It's only that they hate the Republicans more that keeps them under the DNC's tent. The Greens have made better showings than the LP or the Reform party of late, so the concerns over the "spoiler factor" have dwindled a bit.
The knee-jerk "just because" Democrats are aging out of the population, and not being replaced. They also have a lot of overlap with the blue-collar Union wing, and when they do "wake up and look around", they trend socialy conservative too.
The rest of "average Americans" that are the grout between these rocky shoals that make up the Democratic Party. (sorry, mixed metaphor&) Who are they? MSM zombies? Soccer Mommies that think the GOP is "mean to poor people"?
So how do you build a foundation on that? I think it's a house of cards. Lose the "I hate Bush/Rove/Cheney/Rumsfeld" mania, and what is there for a cogent vision for this party?
I'm not saying the GOP doesn't have it's factions. The spend-happy and immigration-weak Congress and current Administration is also alienating lots of voters, however, I can't imagine most of them seriously believe the DNC will address those issues to their satisfaction. That being said, I do think there's still more commonality with the party and it's membership than not, taxes, national defense, pro-economy, "America is Good" etc. Even the pro-life plank isn't that unpalatable to those who are nominaly "pro-choice, but don't like abortion&" and wish it legal, but rare as possible. So I see more "glue" holding the GOP together that's issue-based, rather than a pastiche of left-leaning special interests that comprises the "core" of the DNC.