The sick man of American politics
I'm not entirely convinced that the Democratic party will exist, at least not in the form that we know it, 20 years from now.
People always talk about how American politics are inherently two-party, and I certainly can't argue with the historical record there, but it's also true that those two parties have not been constant over the years. Admittedly, the Dems and Reps have been with us since the end of the Civil War, but even then, they've shifted to occupy different positions on the political spectrum than they did in 1865. Thus, I don't think it's crazy to suggest that another such change would be possible.
Why do I have this opinion? Simple. The Republican Party has essentially dominated political discourse for the past 34 years or so. It's seemed like, as long as I can remember, the Democrats have been fighting a rear-guard action to try to hold back the tide of Republican policy, rather than being able to discuss the issues on their side of the table. Even in the brief interlude in 1992-94 when the Democrats were able to talk about things like national health care and gays in the military, they accomplished depressingly little, being torn apart by their own fractiousness. And those issues are no longer part of the national discourse: health care is a taboo subject these days, and "don't ask, don't tell" is firmly entrenched in policy. I certainly don't see this changing any time soon, except for that rear-guard action becoming more and more ineffectual as the money and media, the ultimate determiners of the victor, continue to tilt to the right. And I haven't even begun to discuss the deleterious effects of the Greens on the party.
Sometimes it seems like the party is only still alive because, like the Ottoman Empire (did you catch the reference at the top?), there's nothing immediately available to replace it. Ralph Nader's dreams aside, there's no way the Greens will ever be a viable party at the national level. Look, the very name of the party is immediately going to alienate the half of the electorate who thinks environmental issues should be subordinate to other concerns, and that's hardly a great foundation to build a national party on. The Green party certainly will continue to show strength in isolated pockets, but that strength is deceptive, since there are still great swaths of the country where the Greens know that there's no point in bothering to run. Barring an unprecedented sea change in American politics, it'll never happen.
Unfortunately, I don't know enough about the politics of the first half of the 20th century to discuss whether a role reversal between the parties can occur again. (If only I could get Mark Staloff to read this...) My intuitive answer is to say no, but then again, I'll bet if you traveled back to 1876 and asked a random political observer if he expected that the parties would end up like they are now, he'd probably think you were crazy. Again, I can't see it happening without a major external trigger; though I have no particular desire to live through a Great Depression of my own, I don't think it's completely implausible, either.
As Perot and Ventura have demonstrated, the wellspring of self-identified centrists dissatisfied with both major parties is quite a deep one, so I suppose the most plausible scenario is the appearance of a similar candidate who manages to secure a surprising victory. I don't find the victory part hard to believe at all; after all, Ventura did manage to do it (yeah, yeah, there's a difference between the gubernatorial and presidential level, I know). It's the question of what comes afterwards. To put it delicately, Ventura and Perot are difficult people to build a party around (after all, the efforts to construct a national Reform party have not really gotten all that far); to put it less delicately, they're both pretty much wackos. But of course, you have to be a little bit crazy to try running a third-party campaign in today's climate to begin with, so we have a true Catch-22.
Thus, I find it implausible that a third-party candidate would win and then be able to construct a new party apparatus from the ground up after winning. Nor do I think the grassroots, bottom-up approach is likely to meet with much success either, since I can't see how such a party could ever find the resources to mount a national campaign. But the scenario I can see happening is a third-party candidate (possibly a Democrat who bolted the party after a bitter primary struggle...) gaining a large number of Democratic defections after election and thus inheriting a large chunk of the party already constructed, and building a viable platform from that. And remember, you heard it here first.
(Okay, you probably didn't. But could you claim that you did, anyway? Please?)