I can't see anyone from the Green party winning a significant national election, as their whole raison d' etre is so micro-focused that they'd have trouble broadening their base enough. The Libertarians might have a better chance, if they either embraced a more hawkish foreign policy or better explained how foreign direct investment and Levi jeans could transform the third world.
In the end, I think the point is that the Democratic party is so split into interest groups with checkbooks, that there is no cohesive set of principles keeping the party together. Maybe a split is inevitable. The big winner, though, would certainly be the Republicans. This seems so obvious, but Eric didn't mention it. The GOP might not gain in actual numbers, and may even lose some if, for instance, the Libertarian party became a more realistic choice, but a fractured Democratic party would scatter its members in a variety of directions. The net result would be marginally stronger GOP. Predictable chaos would then ensue.